Merge pull request #1019 from emanuelschuetze/several-1.5-fixes

Template improvements and translation strings
Some more template fixes:

- Fixed #992 (Use correct countdown format)
- Fixed #909 (Backend und projector template style)
  * Search bar with same border-radius like login button.
  * Projector: Fixed overlay message style. Allow HTML code (use template filter 'safe').
  * Dashboard: Tooltips, Buttons and Widget fixes.
- Fixed template does not exists error if custom slide is activated.
- Countdown: Add css to show negative countdown in red.
- Add missing translation strings to English po file (for transifex).