[api] initial request priority support
[api] new request and review history system
[api] convert old request history DB to new structure
[api] ensure ordering by priority in request search
[webui] use new generic history for requests
[api] add OBS:IncidentPriority attribute

This attribute can be used to define the priority of request inside of projects.
It is supposed to be used in incident projects to repriorize requests.
[api] create also history entries for reviews
[api] set maintenance release request prio based on patchinfo rating