* [x] `=channel <subscribe|unsubscribe>`
* [x] `=stalk <subscribe|unsubscribe>`
* [ ] Subscriptions send you email
* [ ] Subscriptions don't work if you don't have a confirmed email address
* [ ] Removal of email address force-removes all subscriptions, but replacing doesn't
* [ ] Bin mail flag on stalks
XML structure to be done as part of T1232.