[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:
    
 - [x] PR #1395


(Note: I accidentally deleted my branch.. so the old PR got automatically closed. Old comments lives at #1396 .)