Various annotator UI components use rem-based font sizes to try and make the
annotator UI match the document. However various sites (eg. Stack Overflow,
OpenStax) set the root font size to a value that results in the Hypothesis UI
elements having unreadably small or excessively large text. Also for these sites
the rem-size doesn't reflect the actual "default" font size of the page as
perceived by the user.

To resolve this, change the font sizes from using rem units to a value
calculated from a `--hypothesis-font-size` CSS variable. This variable is set to
match 1rem, but clamped to ensure that our UI text is always readable.

Fixes https://github.com/hypothesis/client/issues/4615