Migrate user options to form options

[BurntimeX]

This is a prerequisite for the modernization of various pages/functions:

• [ ] Migrate RegisterForm to form builder
• [ ] Migrate UserListPage to grid view
• [ ] Migrate UserAddForm / UserEditForm to form builder
• [ ] Remove the frontend version of UserSearchForm and allow filtering of MembersListPage instead
• [ ] Remove the admin version of UserSearchForm and allow filtering of UserListPage instead (tbd)
• [ ] Migrate the user profile edit to a form builder dialog