Add ability to reference Lookup groups for access-control

Edit by @rjw57: if people have an opinion on the utility of the feature, please use the "thumbs up" feature to indicate opinions.

We currently use Lookup groups (e.g. https://www.lookup.cam.ac.uk/group/uis-infrastructure) to manage access-control functionality for various systems.

It would be nice if, when granting members access to groups, that we could specify such using the names of existing Lookup groups, and have membership of Lookup groups automatically tracked, rather than having to populate CRSids manually.