Docs cleanups
README: Add more status badges
Docs: Display release notes before changelog

This will reduce the confusion to users.
Cleanup: removed dead code
Cleanup: Remove unused export code
Some cleanups

* Removed unused imports
* Reduce code length
* Improve readability
Cleanup: Use reverse instead of hardcoded URLs
Cleanups from landscape.io report

Missing imports, unnecessary imports, indents, comments...
Cleanup: Remove references to publishing in SourceForge
Cleanup: Remove broken test-publish-pypi

It doesn't work.
Makefile: Remove unused publish directive
Releasing docs: Major cleanup
Templates: implemented generic about page

Brings back stock Pootle content.

Fixes evernote/pootle#262.
Migration cleanup: Specify dependencies just once
Migrations: Rename migration to just have one migration per number
translationproject: Check for disabled projects in more places
User: Inherit PermissionsMixin for our custom User model

AbstractBaseUser, which we inherit from, does not provide "has_perm". This is
taken care of by PermissionsMixin which only AbstractUser has by default.

Incidentally this gives us back is_superuser so we can get rid of it from our
class. No migration needed.
Keeping in mind it *also* brings us groups, which is not so great since we
don't actually use those anywhere.

Fixes #3286
UNDO  not necessary to explicitly require this because is already required