Support custom post types in settings page

The list of post types which users can enable Hypothesis on is limited to Posts and Pages, with a not-well-documented "hypothesis_supported_posttypes" hook which can be used to override this. This means that custom post types added using tools such as [Custom Post Types UI](https://en-gb.wordpress.org/plugins/custom-post-type-ui/) will not appear automatically.

It looks to me like it should be possible for the plugin to enumerate all registered post types automatically using [get_post_types](https://developer.wordpress.org/reference/functions/get_post_types/).

_Context: https://hypothesis.zendesk.com/agent/tickets/1314_