Improve progress indicator on timeslot page

[LukasKalbertodt]

The basics are there, but it needs to be better.

[LukasKalbertodt]

Some possible icons:

• http://fontawesome.io/icon/exclamation-circle/
• http://fontawesome.io/icon/bolt/
• http://fontawesome.io/icon/ban/
• http://fontawesome.io/icon/exclamation/
• http://fontawesome.io/icon/times/
• http://fontawesome.io/icon/exclamation-triangle/