Fixed #1197: Use big heading for custom slides without text.

Keep last projector scale/scroll values after activate new slide.
Added global chatbox for managers.
Improved chatbox style. Updated jquery cookie and jquery ui.

- Fixed js error on dashboard page if no cookie is found.
- Use cookies only until end of session. For dashboard cookies use url /dashboard only.
- Updated permission numbering of builtin groups and added can_use_chat permission.
- Used minified version of jquery.cookie.
- Added TODO comment for django session way from pull request.
Merge pull request #1220 from emanuelschuetze/chatbox

Chatbox (Fixed #504)
Merge pull request #1221 from emanuelschuetze/fix-1197

Fixed #1197: Use big heading for custom slides without text.
Automatic numbering of agenda items
Some coding style changes. Added CHANGELOG and README entries.