New notifications

New notifications

- Added request.notifications
- Email configuration fixes
  - Set config_spec default SMTP port to `0` and switch to SSL/non-SSL
    default if `port == 0`
  - Added email_smtp_use_ssl configuration setting
- Added migrations for notification tables
- Added __repr__ to MediaComment(Mixin)
- Added MediaComment.get_entry => MediaEntry
- Added CommentSubscription, CommentNotification, Notification,
  ProcessingNotification tables
- Added notifications.task to celery init
- Fixed a bug in the video transcoder where pygst would hijack the
  --help argument.
- Added notifications
  - views
    - silence
    - subscribe
  - routes
  - utility methods
  - celery task
- Added half-hearted .active comment CSS style
- Added quick JS to show header_dropdown
- Added fragment template to show notifications in header_dropdown
- Added fragment template to show subscribe/unsubscribe buttons on
  media/comment pages
- Updated celery setup tests with notifications.task
- Tried to fix test_misc tests that I broke
- Added notification tests
- Added and extended tests.tools fixtures
- Integrated new notifications into media_home, media_post_comment views
- Bumped SQLAlchemy dependency to >= 0.8.0 since we need polymorphic for
  the notifications to work