Resolve "Impersonate a user as admin"

This merge request introduces the ability for an admin to impersonate a user. Because the admin user management functionality is a premium feature, this can only be used if a user has access to the premium version. Most of the impersonation logic is handled by the web-frontend.

The impersonation deliberately requires a page refresh. This is because on first page load certain items like groups, application, etc are fetched. Because we don't want to refresh everything, it seemed like a good idea to do the impersonation on page load before this happens.

Mar-14-2022_20-57-14

Testing pointers

• The backend impersonate endpoint can only be used if the requesting user has access to the premium version.
• The backend impersonate endpoint can only be used if the requesting user is staff or super user.
• As much code as possible lives in the premium folder.
• Clicking on the three dots on the admin user management page shows an option to impersonate.
• The JWT token of the admin should never be overwritten.
• Refreshing the page should stop the impersonation.
• While impersonating an orange notification in the top left corner should indicate that you're impersonating someone.
• Clicking on the stop button in the top left corner should stop the impersonation.
• Real time collaboration should work while impersonating.
• A new endpoint must be introduced to the API docs.

Merge Request Checklist

• changelog.md has been updated if required
• New/updated Premium features are separated correctly in the premium folder
• The latest Chrome and Firefox have been used to test any new frontend features
• Documentation has been updated
• Quality Standards are met
• Performance: tables are still fast with 100k+ rows, 100+ field tables
• The redoc API pages have been updated for any REST API changes
• Our custom API docs are updated for changes to endpoints accessed via api tokens
• The UI/UX has been updated following UI Style Guide

Closes #830 (closed)