search queries: allow combining filters with or instead of and

[stuebinm]

Currently in the ticket search, all individual filters will be combined with and, i.e. the search will match only those tickets which match each of the given filters. Unfortunately this makes it hard to search for e.g. all tickets concerning events in any of a list of rooms, or all tickets currently being worked on by a set of encoder.

It would be nice to have an option that combines the filters using or instead.