"allowedSearchTypes" should be moved to the search utility

[brianhelba]

As search logic is now handled in its own utility (in girder.utility.search), the set of allowed model search types (in girder.api.v1.resource.allowedSearchTypes) should be moved out of the API layer and into the search utility.

Ideally, this move would also change this mutable global into a proper internal API.

Since girder.api.v1.resource.allowedSearchTypes is part of the public namespace, it cannot be moved until a major release.

[jeffbaumes]

@brianhelba will you have time over the next week to look into this, at least to scope out the change and see if it's viable to do this for the 3.0 milestone?

[jeffbaumes]

@brianhelba any status update here?