[Bug 787360] Localization for TYPO_SIGNIFICANCE

This commit includes:

 - Backend ignore request to mark ready to localize if the revision is
   of TYPO_SIGNIFICANCE.
 - Reworked tests so that if they want an localization mail, it needs to
   be able to use MEDIUM_SIGNIFICANCE.
 - Hide the option for mark as ready for localization if reviewer
   selects typo significance in the front end.

Depends on:

 - PR #1395