Allow subscribe and unsubscribe to use inbuilt composer

[sebbASF]

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

[Humbedooh]

We would need to answer some questions then:

• What is the subscription method? email a certain address? a CGI form?
• Which address do we allow subscribing to the list? what oauth provides and only that? or other addresses?
• Where would these buttons go in the UI?

[Humbedooh]

Currently, there's a simple subscribe button that opens the user's own email client.

[sebbASF]

It would be nice if the (un)subscribe buttons could fill in the details for the composer, but they don't do so for the user mail client, so I don't see that as essential initially.

It this is implemented, it should also apply to the client email launch. There's no point launching an email client at all unless (un)subscribe can be done using email, so I don't think we need to consider using a CGI form.

The composer option should allow the user to select any of the users authorised emails, as was done for classic PonyMail. However AFAICT user preferences are not yet supported.

In PonyMail classic the buttons were at the bottom of the thread list, which worked OK for me. There's more room there than under the calendar.