Convert gittip.css to a dynamic simplate

Instead of depending on `make css` to rebuild css, this calls scss
dynamically whenever gittip.css is requested.
Add a CSS_HREF config for finding the CSS file

This lets us use the production file in local development, parallel to
how we use a cloud db at first.
Provide for versioning in CSS_HREF
Add CSS_HREF to default_test.env

Thought I had done this. Oh, well.
Merge branch 'master' into pipeline-css
Be strict about what asset versioning
Install sass using a post_compile hook

This hook is part of heroku-buildpack-python.

ht @btubbs and @kennethreitz
Merge branch 'master' into pipeline-css
Tweak version of CSS in dev

I changed the gittip.css simplate to 404 if the version requested doesn't
match the current version of the app, so that we don't accidentally
serve CSS that's out of sync with our markup. Without this we'd actually
have a potential attack vector, in that someone could prefill our CDN
cache with the current version of the CSS by requesting future versions
from the CDN that would be served with the current version by the origin
server.

For development we don't want to require sass out of the box, and so we
want to be able to point to a CDN-hosted CSS file by default. Since the
version in development won't match the version in production, I added an
out: if you specify '-' as the version you won't get a 404, you'll get
the file. This changes default local.env to take advantage of that.
I guess we don't have indent here?
Bring back the cache_static module

We had a cache_static module but I turned it off because it was
misbehaving: https://botbot.me/freenode/gittip/msg/4196280/. I'm
bringing these hooks back because it looks like we need to configure
cache headers to make it work with MaxCDN. There are facilities in the
MaxCDN dashboad for working around various issues, but they don't appear
to be working for me, and I'm getting less than stellar response from
the on-page live chat over there. So this is me trying to change the
headers at the origin so we start seeing cache hits at MaxCDN. I've set
up a pull zone for QA at MaxCDN so we can fully test this front to back
before deploying to production.
Let's add the indent function to post_compile
Fix bug where non-versioned assets were 404
Update some docstrings for cache_static
Constrain caching to /assets/ instead of /assets

What if someone has a username assets-oh-yeah-assets?
Standardize logic for inbound cache hook
Only cache things under %version
Fix regressions in request API usage for caching
Merge branch 'master' into pipeline-css

Conflicts:
	www/assets/%version/gittip.css
Provide better error message when envvars missing

Fixes #1163.