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.
Ok, I will change this and other occurrences in the current file. However, for the rest of the code, I will submit another PR.

This was to select the default calendar in dropdown list.