Bug 1082853: Configure CSSLint and validate MDN CSS

Made a more decerning vendorize mixin with lookup tables for values.
Made a mixin for creating rgba fallbacks.
Remove table.activity which was presumably added back in during a merge somewhere).
Validate CSS, mostly working with outlines.
fix bug 1088403 - add bn-IN locale
fix bug 1087527 - use the SessionAwareClient in more tests

This seems to have been a leftover from 5743ed2059ccd4dc02ed7ccb789d2f637c2dfc2f.
fix bug 1072380 - make sure we raise validation error at the right spot of the call chain.

The problem was that the validation error was raised at a stage where it didn't trigger a re-rendering of the page.
This also contains some refactoring of the newsletter feature to encapsulate it in the NewsletterForm.
Merge pull request #2840 from stephaniehobson/BUG-1082853-csslint

Bug 1082853: Configure CSSLint and validate MDN CSS
bug 1082853 - Remove stylus vendorize warning
Merge pull request #2857 from groovecoder/add-bn-IN-1088403

fix bug 1088403 - add bn-IN locale
Merge pull request #2845 from jezdez/bug1072380

fix bug 1072380 - make sure we raise validation error at the right spot of the call chain
Merge pull request #2862 from darkwing/fix-vendorize

bug 1082853 - Remove stylus vendorize warning
fix bug 753563 - Support username changes

Users now find an extra field on their profile edit page titled `Username`. Changing this
field allows them to change their username.
Existing usernames that do not comply with the new validation rules will not be affected.
fix bug 1087577 - show edit button on profile page for staff users

This also shows a warning on the edit profile page if the profile's user isn't the same as the editing user.
bug 753563 - move username regex and error message to central location.
fix bug 1091017 - get rid of Django < 1.4 shim for DatabaseError.
fix bug 1090241 - get rid of PlusToSpaceMiddleware
Merge pull request #2846 from jezdez/new-issue-753563

fix bug 753563 - Support username changes
bug 1072493 - Add wiki UI tests