Adjusted layout for side-by-side views

Several changes to better fit short messages when displayed in a
side-by-side view such as page or proofread:
* Margins are used when width is more than 900px
* Style adjusted to make the list of messages look more like a document.
* Fixes in the buttons to switch views to make them look connected.
* By using a tux-view-switcher class, CSS rules for addign icons on toggle buttons are smplified.

Change-Id: I39a36df9d76b10dad5c511be391d20345e84b663
Merge "Adjusted layout for side-by-side views"