Allow subscribe and unsubscribe to use inbuilt composer

If the user is logged in, it should be an option to use the Ponymail composer to subscribe and unsubscribe.

Also if the user is not logged in, start new thread should offer to start the user's email client

Start by locating the subscribe and unsubscribe entry points, the inbuilt Ponymail composer, and the Start new thread behavior. Check the logged-in and logged-out flows, then verify that authenticated users can compose subscription changes while unauthenticated users are offered their email client.

Written by the indexing model from the issue text.