Ajaxify contact form

Partial pick of evernote@a76f6577.
Use contact form to report string errors

The new form serves both as a way to contact site managers as well as to
report errors with strings from the editor.

This also renders useless the need for a `report_target` field in
`Project` models, so that field has been deleted too, along with its
related functionality.

This, and some other previous commits, fixes evernote#15.

Partial pick of evernote@a76f6577. The rest of the original commit is in
several previous commits.
Cleanups on fields
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).
Refactor contact app to report string errors to project report email if exists

If the project has no report email, then fall back to a global report email.