endpoints for reading/updating user's subscriptions

backend support for this: https://github.com/kir-dev/konzisite-frontend/issues/170
get endpoint to get all the groups and subjects you're subscribed to
maybe a post/patch endpoint for updating these in bulk
or each subject/group can be changed individually with the existing post endpoints

Start by reading the linked frontend issue 170 and inspecting the existing POST endpoints for subscription changes. Determine the expected read and update behavior from those references; the work is done when the backend supports the agreed subscription retrieval and update flows.

Written by the indexing model from the issue text.