Minimum and maximum identities of the same type in a user

* The admin can set minimum and maximum identities of a user.
* For example, if every user must have minimum 1 and maximum 3 email addresses:
* The end-user/admin cannot remove their last email address
* The end-user/admin cannot add more email address if there are already 3 in their account

No files, tests, or entry points are named. Start by locating the identity-management paths that add and remove identities, then trace how per-user identity limits are configured and enforced. Done means minimum and maximum counts are enforced for both end-user and admin actions, including the email examples in the issue.

Written by the indexing model from the issue text.