Categorise members by role in the ChangeRoles screen

[jmartinesp]

In Settings -> Roles and permissions -> Admins or Moderators we want to have a clearer distinction for which members have which roles.

Changes needed:

• Add different sections for the 3 main roles: Admins, Moderators, Members, in that order, and display the users in each category.
• For users whose role can't be changed display a disabled checkbox instead of an empty space in the right of the list item.
• If the user is in a pending state (invited, not joined yet) display a 'Pending' label next to their display name and before the checkbox. Designs are pending for this at the moment.

See the parent issue for more details on this.