Inset view controller for inset elements of forms.

Summary:
T937 suggests 'inset' could have its own view controller.

It has the following methods:

• setTitle for title
• setRightbutton if you have to place something (preferably a button) on the right side of the form
• setDescription if you want to describe what it does
• setContent for the main content
• addDivAttributes REALLY not sure about this one but it had to be included because of a single controller (see owners/controller/edit/PhabricatorOwnersEditController.php:238)
• appendChild works as usual if your form is complex but you still want to remove ->appendChild('<div class..') ->appendChild('</div>');

It might be an overkill so maybe some could be dropped:

• addDivAttributes() and just rewrite how PhabricatorOwnersEditController.php works
• setContent() and use appendChild for the main content?

Test Plan:

• Looked at the controllers in phabricator
• Changed the controller
• Opened the page in another tab
• If something didnd't look the same I fixed it.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1926