Add endpoints for listing, adding and deleting permissions

[felixridinger]

At the moment, the contaxy API does not allow to directly view and modify the users permissions. It's only indirectly possible (e.g. adding a user to a project adds the permission for that project). For extensions or admin users it would be helpful, to have direct access to the user's permissions. Therefore, API endpoints for the following actions should be added:

list permissions of a user add a new permission to a user remove a permission from a user list all users that have a certain permission

[lgtm-com[bot]]

This pull request fixes 1 alert when merging d5d1b1f635a44068a77247a03551788c6d33fe1e into d98b547c3ff72fe3fb1d1d031322b77d5bf375fd - view on LGTM.com

fixed alerts:

• 1 for Non-iterable used in for loop

[lgtm-com[bot]]

This pull request fixes 1 alert when merging e50401be4134818cb6df7764753860dcf944fe32 into d98b547c3ff72fe3fb1d1d031322b77d5bf375fd - view on LGTM.com

fixed alerts:

• 1 for Non-iterable used in for loop