Allow custom Templates for hooks (BB-8842)

I would be very helpful if the messages for each service can be edited using a template with certain placeholders for the dynamic information.

For example we use the HipChat service. But we don't use the HipChat web client, we use Jabber clients. So we see all the HTML in the Jabber group chat which is ugly. We could fix this if we would be able to modify the messages send by the hook.