Let users set a default calendar for new events [with hidden buttons]

:construction: 
Continuing from: https://github.com/owncloud/calendar/pull/399

I will need @jancborchardt's help at this stage since I am not a CSS-friendly guy :). Also if @Raydiation can help me this would be perfect.
PS: you should probably review the template code (and every print_unescaped). You should only use print_unescaped if you want to include html