fix bug 1034189 - remove notifications app.

The app is a left over from before django-tidings replaced it.
Bug #1070001 - Re-organize stylus

- Switched from @import to @require.
- Files should now require the files they need rather than relying on an @import in a different file.
- Files in /include should not generate output.
- Tried to break files in main into sensible smaller pieces.
Bug 1070001 - Re-org Stylus - Tags

All tags now use same class ".tags".

There is a modifying class ".tags-small" for the tags in the wiki footer.

There is a .tags_tagit file to extend this with over-rides for the jQuery UI plugin.

Things that are not tags (list of interests on profile page) no longer use the class ".tags".
Merge pull request #2777 from stephaniehobson/BUG-1070001-reorg-stylus-tags

Bug 1070001: Re-org Stylus - Tags
Bug 1070001 - Reorg Stylus - Activity

Moved .activity into its own file and changing class names to reflect component structure.

Also removed .actions reference from API key page (there were two different uses for the .actions class, but turns out it wasn't doing anything on API page, so re-styled that page to use a different component).

Pages to test (you will have to create some API keys):
https://developer-local.allizom.org/en-US/profiles/stephaniehobson
https://developer-local.allizom.org/en-US/users/account/keys
Bug 1070001 - Reorg Stylus - Light Background

Convert all instances of #f4f7f8 into $light-background-color.
Merge pull request #2802 from jezdez/bug1034189

fix bug 1034189 - remove notifications app.
Merge pull request #2781 from stephaniehobson/BUG-1070001-reorg-stylus-activity

Bug 1070001: Re-org Stylus - Activity
Merge pull request #2807 from stephaniehobson/BUG-1070001-reorg-stylus-light-background

Bug 1070001: Reorg Stylus - Light Background
fix bug 866542 - Hide attachment table on document display page