Previously the overflowing state of the excerpt was computed implicitly
by the overflowing() method, which was called from several other
controller functions.

There were several problems with this approach:

 1. Since this method calls Element.scrollHeight, it triggered an
    expensive synchronous layout flush.
 2. In certain situations, this could trigger a loop where the
    overflowing flag was continually flipped (see #2960)
 3  It was unpredictable when the overflowing state would be recomputed.

This commit instead opts for a different approach which is more explicit
about when the state is recomputed, based on the assumption that the
<excerpt>'s content size is a function of:

 1. The inputs to the <excerpt>'s content (eg. the text of a quote, the body
    of an annotation)
 2. The sidebar window's size
 3. The dimensions of any embedded media

The content size is then computed and updated as follows:

 1. When the <excerpt> is initially created, an async computation of
    the scrollHeight is scheduled. The calculation unfortunately needs
    to be async because the content may contain Angular directives which
    take several $digest cycles to fully process.

 2. The <excerpt> listens for changes to the content's data (via a
    'contentData' property), media 'load' events and window 'resize'
    events and recomputes the overflow state in response to any of
    these.

There are a couple of other fixes here:

 - Use an optional boolean property for the 'enabled' input,
   rather than a function. The app passed a function to this property
   but the tests passed a boolean.

Fixes #2960