Give users more information why the quality check has fired and why it
is important to fix this problem.

Fixes #417

There is a special rule when the translation is empty, it is treated to
be same as source string.

Fixes #420

Fixes #418

Currently translated at 98.9% (963 of 973)

This gives user better visual feedback what went wrong.

- load once we're getting close to bottom of page to prevent user
  waiting for units
- prevent multiple loads at one point

- We don't need to query lock state in advance
- Update user lock while editing in zen mode

Now only 20 translations are loaded and additional are loaded
dynamically as user scrolls down.

Issue #375

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

Issue #375