WIP convert to role-select overlay on cp users #2761

**Note: WIP Do not merge. Breaks manage members page**

#### Changes
* Remove old "add-admin" & "remove-admin" actions
* Consolidate API functions that generate table
* Add new "edit-membership" action
* Remove non-standard "user_id_load()" function in favor of user_load()
for menu item object loading
* Correct user-facing text: group members are "members", not "users".
@wip add options to edit membership form #2761
debug role add/revoke #2761
verify edit role workflow and add css style #2761
clean up code for review #2761
standardize var names and end user text  #2761
Merge pull request #2999 from openscholar/2761-edit-roles3

Update CP > Users page to allow for 3 available site member roles #2761