Move db:setup part to "Initialize db configuration" section

As pointed out by @menor, this step requires that you have already
initialized the database.

Fixes #114.
Start depending on published diffux-core gem

I just pushed version 0.0.1 of the diffux-core gem. Instead of depending
on the github repo itself, we're now using the latest version of
diffux-core.
Fix ViewportsController#update redirect

In fdd9db6d5f5cbc0566 I i18nalized this file, but I made a mistake when
converting the redirect. I forgot the `:notice` key. This commit fixes
that problem.

Change-Id: I74d93617ec18f1121cce19725d15e44a5b543b87
Fix SweepsHelper#sweep_status

When I i18nalized the helpers, I made a couple of errors that caused
this spec to fail.

Change-Id: I2f3c5fd39f1fd6b39689caa7430d29512b20d2bb
Update aws-sdk 1.38.0 -> 1.39.0

Change-Id: I39c0b6f4fcd74351d2309f84499dc5bcbb215d32
Update chunky_png 1.3.0 -> 1.3.1

Change-Id: I950cf7d0440b1bb83e15a2c1b327ba719fa41cee
Use i18n for projects/new intro

I had already extracted this to our config file, but failed to use it.
This commit resolves this problem.

Change-Id: I22d60f732c86f8c9a5a0547b2b809f2de3ce6936
i18nalize about and viewports/edit

As @menor pointed out in #116, there were a few more opportunities for
extracting copy from the app into our en.yml configuration file. This
commit attempts to take care of most of these.

Change-Id: Ibf152d6be2e3f9f8322520647d0d94cf85e43cc3
Merge pull request #115 from menor/spanish_translation

Spanish translation
Set up URL-based locale switching

This makes switching the locale of the app as easy as changing the first
part of the URL. By default it puts you in en. We currently have a
Spanish translation which you can switch to by changing the "en" in the
URL to "es".

We will probably want to at some point add an easy way in the app to
jump to a different locale, but in the meantime this seems pragmatic.
Also, instead of defaulting to en, it might be nice to extract the
locale from the HTTP_ACCEPT_LANGUAGE header and redirect there to begin
with.

Change-Id: I9432f3da7cfbe4cf95248aab4c57b11d7b793dc0
Add Sidekiq dashboard

We are using Sidekiq as an asynchronous task queue, and it has a pretty
good dashboard. This commit makes the Sidekiq dashboard accessible via
/sidekiq.

Change-Id: Ic859b88a0eb9794602bba7be2f54c0092c22157b
Un-scope refresh route

I recently scoped all routes under the locale. Well, the refresh route
is referenced by JavaScript to auto update snapshots and progress bars
that are waiting to be finished. This route always returns a JSON
response that includes no localized copy, so we can just un-scope it to
make it work again.

Change-Id: If446e67da0013223f0139a9982272289b993c6c8
Allow locale in URL to be optional

When I first added URL-based locale specification, I made it required
and had `/` redirect to `/en`. However, I realized that it would be
better to just make it optional. This will allow routes like refresh and
sweep triggering to work as they did without any change, but to also
optionally specify a locale.

Change-Id: Ib75ffe8bb4c2396dd41f9e89d788ed368255682c
Switch to latest diffux-core

I had to fix a critical bug in diffux-core that was causing snapshotting
to fail [1]. Switching back to latest diffux-core for a while until
things have stabilized.

[1]: https://github.com/diffux/diffux-core/commit/8d46508bba1b376e16cd2329b2358397682366e9
Fix missing translations for timestamps

I was trying out the Spanish translation (awesome btw!) and noticed that
the snapshot timestamps didn't all have translations. It turns out that
`distance_in_words` has a total of 11 variations, and we only specified
three.

I first attempted at using my high school Spanish to fill in the blanks
myself (hey, I had a an A in Spanish after all). But I realized that I
had no idea what I was doing. Then I came across the rails-i18n gem
which provides default translations for a large number of languages. It
turns out Rails is smart enough to merge multiple locale files together,
so I was able to rip out some stuff from our es.yml.

I did notice that the translations in the rails-i18n for
distance_in_words didn't match the ones that @menor had provided. So I'm
going to ask him to verify that the default es.yml is doing the right
thing:
https://github.com/svenfuchs/rails-i18n/blob/master/rails/locale/es.yml
Update diffux-core 0.0.1 -> 0.0.2

Change-Id: I2895635e55e842ab8e9dff61f515d6eabb193acd
Fix sweep progress bars when using es locale

We're using the `number_to_percentage` helper to construct the
css/style string used to control the width of the individual bars inside
the progress bar. This method is sensitive to locale, so when in "es"
locale, the delimiter would be a comma instead of a dot. This caused
invalid css. I've fixed that by passing in an "en" locale to the helper.
Bring back removed distance_in_words strings

I removed these thinking that we didn't need them. @lencioni pointed out
that he had added them explicitly to override some default strings that
were too long. I didn't think of that when I removed them in
350b9408813015393cddba, so this commit will bring them back again.
Remove extra period from Spanish translation

I was browsing around Diffux in Spanish and I noticed double
punctuation. This commit fixes that problem.

Change-Id: I4d1d70db552e71262619b8a4639c9029ab412aa4
Actually update diffux-core gem

When I bumped the gem version, I failed to commit the Gemfile.lock
changes.

Change-Id: Ib489cebabc78ce2d9fa68832b67821fcfa9256f6