Enable publishers to customize the user interface font in the Hypothesis client

We currently enable publishers to customize the font family used to render annotation text and quotes in the sidebar via the `annotationFontFamily` and `selectionFontFamily` config params, in order to make the client better fit the look and feel of their site.

This accounts for most of the visible text in the sidebar, but not other elements of the UI such as the top bar. We have had a request for publishers to be able to customize the base "user interface" font that is used for most other elements of the sidebar.

Start by locating the existing handling of the annotationFontFamily and selectionFontFamily configuration parameters and the sidebar styles that define the top bar and other UI text. Confirm which elements use the base user interface font, then define a publisher-configurable font that applies to those elements and verify that annotation and selection fonts remain separately configurable.

Written by the indexing model from the issue text.