Allow plugins to delete messages

Allowing plugins to either delete individual messages or all messages of their respective application would add significant flexibility.

To be able to delete individual messages the `SendMessage` would also have to return some kind of message ID, so that the plugin knows which message to delete.

Together with https://github.com/gotify/server/issues/266 this could be utilized so that a plugin can discard a message on all connected clients given some certain event.