Audit defaultValue: null for all config defs in config/index.js

For legacy purposes, we kept `defaultValue: null` for most of the config definitions after a [recent refactor](https://github.com/hypothesis/client/issues/3236), but it's quite possible we can remove this default and simply allow the setting to be omitted from the returned config.

https://github.com/hypothesis/client/blob/0de918ff9d62b950cc03c985aaf405184a6ae9fc/src/annotator/config/index.js#L86

A deep audit of these settings and consequences of removing the default should be done. In all likelihood, many, if not all of them may be able to not need a `defaultValue` anymore. Some changes may need to be done in the sidebar to handle a missing value as opposed to a null.