EN auth: type cast before checking cookie expiration date
Add `ads` word to ignore list in `date_format` check

Fix #3423
EN checks: use an exception to skip check instead
EN checks: use any symbol except `}` instead of non-space symbol to define mustache placeholders.
EN checks: split mustache_placeholder_pairs check into two to properly deal with non-strict pairs

Fix #3422
EN checks: add more tests
Recover `fuzzy` state for obsolete units after resurrection

Fix #3421
Reports: add references to user translations from user stats page

Related to #3432
Extend mustache placeholders check with `{<number>, <xxxx>}`

Fix #3434
Permissions: operate on copies of module-level mutable data

Be careful not to shoot ourselves in the foot.
Cleanup: DRY
Travis: no need to set cache backend settings

These are already set in `tests/settings.py` which will be used when tests
are actually run.
(WIP) Languages admin: revamped functionality

Implements the new admin UI for languages