Contacts App Miscellaneous Style Fixes

The PR fixes the following:-
<ul>
<li>The headings in the left content get centered.</li>
<li>The Add Contact UI gets all labels and inputs together, doesn't look messy anymore.</li>
<li>All buttons are centered to the rows.</li>
</ul>
This PR doesn't fix :- 
<ul>
<li>The Arrow in Add Contact UI.</li>
<li>The Delete / Close Button Confusion.</li>
</ul>
Separate PRs for the above.
Ref: https://github.com/owncloud/apps/issues/1033
/ cc @tanghus @jancborchardt