Create user profile endpoint that provides user's groups and institutions

Resulting from !87 (merged), removing lookupproxy means that some functionality used by the frontend (particularly institutions) is now missing. Add a profile endpoint to replace this.

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mentioned in issue frontend#178 (closed)

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marked this issue as related to frontend#176 (closed)

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Superseeded by https://gitlab.developers.cam.ac.uk/groups/uis/devops/iar/-/epics/1

closed