Management API: bulk template access for agency users

You can now manage which templates your agency users can access through the Management API, for many users in one request. Template access restriction was already available in user management, and the API could already set it for one user at a time.

Send a list of users to "POST management_open_api/agency_users/template_access" and pick a mode for each. "replace", the default, sets their access to exactly the templates you list. "add" and "remove" change only the listed templates. A new "GET management_open_api/templates" endpoint lists your agency's templates and their IDs, and agency user responses now include each user's template IDs. Each request is validated as a whole: if any user fails, nothing is saved.