New Nginx config to automatically append slash.

The Nginx config needs to handle 3 cases for serving files:

  1. /en/latest/file.html   -> Serve file.html
  2. /en/latest/            -> Serve index.html
  3. /en/latest             -> Redirect to /en/latest/ (append slash).

Previously, the Nginx config handled the first 2 cases, but didn't
handle the 3rd case very well. In that case, it served up the
index.html page without redirecting, which would break relative links
(See issue #644.)

This was due to my misunderstanding of how the index directive worked
with try_files. I thought that "try_files /translations/$uri/" would
do an internal lookup in the filesystem of the /translations/$uri/
directory, and try to serve the index.html page. However, what it really
does is send a redirect to http://readthedocs.org/translations/$uri/.

Needless to say, that URL breaks. To mimic the directory lookup, I had
added rules like this: "try_files /translations/$uri/index.html". But, even
though it served up the index page, this was pretty kludgy, and as mentioned
above, didn't properly append a slash and redirect.

The new Nginx configuration in this commit sets separate locations for
/translations and /single_version, uses a try_files directive that handles
all 3 cases above, and falls back to Django for any 404 (NOT FOUND) and 403
(FORBIDDEN) error.

Why fall back for 403 errors?

Because a URL like http://pip.readthedocs.org/en/ will cause Nginx to look
for /en/index.html. When that's not found, and Nginx doesn't have permission
to list the directory, it will return a 403. But, we would rather pass the
request to Django, so it can generate a redirect to
http://pip.readthedocs.org/en/latest/.

Fixes #644.
Remove special case for appending slash.

No longer need a special location directive for "/en/latest$", since
appending slash and redirecting will now be handled for all directories.
Keep @fallback consistent.

Set "proxy_buffering off" for @fallback for CNAMEs, just like it's
set for the other @fallback location directives.
Add append slash test to nginx-smoke-test.py.
Fix error_page directives for CNAMEs.

They should be consistent with the subdomain ones.
Move rtd_tests/tests/base.py -> rtd_tests/base.py.

First step in reorganizing the rtd_tests directory so that only
tests are in the rtd_tests/tests/ directory.
Move mock_api.py into new rtd_tests/mocks dir.
Move rtd_tests/tests/files/ -> rtd_tests/files/.
Ensure postgres is running, not just enabled.

In salt/postgres/init.sls, replace service.enabled with
service.running.

Fixes #695.
Ensure that only public projects are displayed on a person's profile.
Fix comments for Project full_path methods.
Fix comment for rtd_build_path().
Use base.html template for 404.html.
Use PRODUCTION_DOMAIN for 404 header links.

Inline header HTML into 404 template.

Fixes #703.
Move 404 header block to error_header.html.
Update 500.html to use base.html.

Give 500.html the same structure as 404.html. Use base.html
and error_header.html. Keep ASCII Fail Whale!
Merge pull request #702 from toffer/fix-projects-models-docstrings

Fix docstrings in projects/models.py.
Fix logic around Project permissions.

We shouldn't be filtering skipped projects while checking permissions.
This was cauisng weird bugs in the UI.
Make skip properly skip non-HTML builds.
Explicitly set project_slug for default kwargs