* Remove Google Plus share options from the client

- Google Plus is shutting down, so remove share icons for it from the
  client
- Add padding around remaining icons in annotation share dialog and
  spacer after "Share" label to avoid having too much space between
  icons

Fixes https://github.com/hypothesis/product-backlog/issues/947

* Update icon font to remove Google+ icons

Remove the Google+ icons and regenerate the icon font using the Icomoon
app, following the process described in src/styles/vendor/ICOMOON.md.

In the process I had to select the option in the Icomoon UI to remove
custom colors from many of the icons. In the client the
`h-icon-*::before` selectors should specify only the font to use and
content to show but not a color. The color is inherited from the parent
elements. I believe this is necessary because it has been a long time
since the icon font was last updated and the Icomoon app has evolved
since then.

* Add notes about how to update or remove icons from the icon font