Docs: Add release notes for Pootle 2.5.2-rc1

This will allow to record changes for next release using specific release
notes instead of using the changelog (planned to be removed).
Cleanups on fields
Minor cleanups on views, templates and url patterns
Improve Django 1.5 support: Djblets imports cleanups

Avoid using deprecated code to allow using latest Django releases
Cleanup: unified admin URLs
Unified names for named URL patterns
Cleanup: removed dead code

This code path was never reached.
Rename URL pattern to align with Evernote
Reposition URL pattern to align with Evernote

This eases picking their changes
Wrap URL pattern

This eases picking Evernote changes
Cleanup: unified and cleaned URLs
Fixed URL pattern name
Name URL patterns
Use url template tag
Cleanups: Use URL names and url template tag
Cleanup: coding style fixes
Cleanups: Renamed vars and other minor changes to align with Evernote

This eases picking the next commits.
Cleanup: gathered templates in a single location
Cleanup: Fix template references in templates that we have and Evernote doesn't
Cleanup: Move remaining templates to pootle/templates/

Also moved templates from pootle/templates/common/ to pootle/templates/core/

This gets Pootle working again since the app templates loader was removed in
the previous commit.