Better logic for loading content in zen mode

- load once we're getting close to bottom of page to prevent user
  waiting for units
- prevent multiple loads at one point
Sync end of translation message
Basic tests for zen mode
Ensure unique element ids on zen page
Commit pending changes before setting commit message
List failed checks in error message

This gives user better visual feedback what went wrong.
Merge branch 'master' into zen
Move checks logic together
Move fixups logic to views
Move redirect logic to single place