Fix an issue where elements in the annotation body that were styled with
margins, such as headings and block quotes, could cause the bottom of an
excerpt to cropped.

The issue was that margin collapsing [2] caused the margin to "leak" out of
the excerpt's content `<div>` and push it vertically away from the outermost
element of the excerpt. The `max-height` applied to the outermost
element took into account the content's _height_ but not its vertical
offset.

The fix here is to make the excerpt content div a new block-formatting
context [1] which ensures that any margins on elements inside the
content are placed _inside_ the content element. There are various ways
of doing this, but `display: inline-block` is backwards compatible down
to IE 11.

 - Fix margin collapsing issue by making content a new block formatting
   context
 - Remove unused `excerpt__container` class and an incorrect comment
   about the excerpt class
 - Remove an unnecessary `test-name` attribute on the content element and
   use the class name to find it in tests instead
 - Add a note about some refactoring that can be done post SASS-module
   adoption

[1] https://developer.mozilla.org/en-US/docs/Web/Guide/CSS/Block_formatting_context
[2] https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Box_Model/Mastering_margin_collapsing

Fixes #1518