Initial read only zen mode

Preparing support for zen mode, right now it's simple read only listing
of strings.

Issue #375
Edit forms in zen mode

Issue #375
Display more unit information in zen mode (issue #375)
Include context information in zen mode (issue #375)
Dynamic loading of translations into zen mode

Now only 20 translations are loaded and additional are loaded
dynamically as user scrolls down.
Properly indicate end of translation in zen mode
PEP8 compatibility
Locking in zen mode

- We don't need to query lock state in advance
- Update user lock while editing in zen mode
Merge branch 'master' into zen
Merge branch 'master' into zen
Merge branch 'master' into zen
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
Merge branch 'master' into zen
Move checks logic together
Move fixups logic to views
Move redirect logic to single place
Remove extra blank line