Generalize vacation handling

Vacation handling in James is tightly bound to JMAP, where it is a mandatory part of the specification. However, this feature is actually of general use. It should be possible to manage vacation settings through other means as well, such as the WebAdmin interface. This also enables easier integration of James with third party services, e.g. a web admin GUI.

For this purpose, the vacation/notification storage layer and the VacationMailet should be refactored out of JMAP space, so they can be used in an independent manner.  It is then possible to add WebaAdmin routes to query and change a users vacation settings.

T-Shirt size L (package changes affects a lot of other code)