Various cleanups to Ubuntu installation instructions:

* Avoid editing /etc/apt/sources.list add nginx repo, instead create
  /etc/apt/sources.list.d/nginx.list

* Avoid editing /etc/nginx/nginx.conf, instead put server_name_hash_buckets
  setting into discourse.conf

* Proper-case Discourse when it is referring to the product name, so that all
  the lowercase references are to the user created during the installation
Address @Supermathie's concerns in PR1430
Updated Japanese locales
Optionally allow discourse to create new topics from email.
Refactor and clean up New-Topic via Email

With the new email_in admin configuration setting, emails to the email_in_address fetched via POP will now be processed and posted as new topics to the forum.

With the email_in_min_trust you can control the trust level the user needs to have at least to be able to post an email as a new topic.

Also contains tests for the email-in feature and minor clean ups
Add Email-In-Per-Category

 - allow the configuration of an inbox-email-address per category
 - post emails to that email into that category instead of global

 - Adds UI for configuration
 - Adds Documentation for configuration
 - Adds Tests for new feature
Add public-inbox to Email-In-Feature

 - Adds the advanced option to accept email from non-users per category email-address
 - Adds tests covering the new feature
 - Adds UI to configure this feature in the frontend
Switch to proper exception handling system for better user feedback

 - Replace implicit return code-system in Email::Receiver with proper exception system
 - Update tests to check for exceptions instead
 - Test the PollMailbox for expected failures
 - Add proper email-handling of problematic emails
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Dropping squashing commits from contribution process

 - Rephrases squashing of commits before submitting from a requirement to a recommendation
 - minor fixes around that
FEATURE: better UX when backing-up the site

Users won't be prompted whenever the site goes into read-only mode.
Instead, a banner will be shown at the top of every pages.

Also, the browser won't automatically reload whenever read-only mode is
disabled. This really was bad UX.
OPTIM: change Top Topic's score computation algorithm

New algorithm will not wipe the entire table anymore and will only remove
invisible topics and add new visible topics.
Version bump to v0.9.8.8
Upgrade ember-cloaking to not trigger scroll events when the back button
is hit.
Make the back button less awkward when leaving a topic.
BUGFIX: Fix issues if the DOM element is destroyed while the interval
is ongoing (in particular this was weird for Firefox)
FIX: moderators should not be able to see site setting changes in the staff action logs. Fixes #2027
2gb ram more strongly recommended
add better new forum image
hey valid HTML is always a nice bonus
Explicit button text in abandon dialog in all lang

The current post abandon dialog displays typical yes/no buttons and should
instead show more informative button text.

This commit updates all `config/locales/client.*.yml` files and *alters the
structure* of the `post.abandon` node, and *adds three new nodes* under it:
`confirm`, `yes_value`, and `no_value`. The current value of the
`post.abandon` node is moved into `post.abandon.confirm` for all languages.
For English language text, the nodes `post.abandon.yes_value` and
`.no_value` are changed to `"Yes, abandon"` and `"No, keep"`, respectively.
For other languages, the standard yes/no values are copied, allowing simple
copyedits to make them more explicit in the future.

I also make the necessary changes to the `composer_controller.js` file so
the new values are used correctly.