Add new status-button directive

This adds loading/success messages to a button for use with async
actions such as changing account information.
Add status button to assets.yaml
Update attribute names for the status button
Use camel case for angular event names
Move the status button setup into the compile phase
Update the status-button to use the compile step
Use transclusion for status-button directive

Using transclusion allows for the directive to be set on an element
with non-trivial content and have that content nested inside the
status-button wrapper spans, but pre-bound to its lexical markup
scope.

Not sure we'll need it here, but I also wanted to see how one might
approximate 'replace' now that it's deprecated. The answer is by
not replacing but just adding the template you want after the passed
placeholder comment node.
Merge pull request #1359 from hypothesis/button-indicator

Add directive for providing saving updates to buttons