Convert the `Excerpt` component to Preact. Rather than convert the
existing implementation verbatim, this is a completely new implementation which
should be easier to understand and use. Instead of requiring callers to
provide an input property which represents the displayed data, which triggers a
re-measurement if it changes, the new implementation observes the DOM
directly for size changes.

This component renders caller-provided content (ie. it accepts a `children`
prop), which is not supported by the Preact <-> Angular bridge. Therefore it was
also necessary to create components (`AnnotationBody`, `AnnotationQuote`) that
encapsulate uses of `Excerpt` inside the `<annotation>` component.

 - Add `observe-element-size` utility module to watch for changes in
   the size of a DOM node using APIs available in the current browser

 - Add new `Excerpt` implementation and remove the old one

 - Remove `excerpt-overflow-monitor` utility that is not used by the new
   implementation

 - Add `AnnotationBody` component to render an annotation's markup body
   inside a (new) excerpt and convert the Angular template for
   `<annotation>` (annotation.html) to use it.

 - Add `AnnotationQuote` component to render an annotation's quote
   inside an excerpt and convert `annotation.html` to use it