[UX] Provide an indication for the default admin/editor roles (if configured)

[klonos]

Since the fieldset that allows to configure these two special role functionalities is collapsed by default, I would like us to be indicating whether a role is set as the default admin/editor in the role management page.

Ideas to consider:

• take advatange of the descriptions/hints for fieldsets
• use the same patern that we are currently using to indicate that the anonymous and authendicated roles are special (the (required) suffix to the role name)
• possibly warn that the role has special functionality when deliting one of these roles (in the role deletion confirmation dialog)
• possibly indicate if a role is special in the role edit form
• possibly allow setting a role as the default admin/editor if not already set

[klonos]

...something like this perhaps?:

[klonos]

...and this:

[klonos]

...each time I have to deal with confirmation forms, I'm being reminded of #3422 and the need for #3329 as well. I'll leave this as "needs feedback", and go and work on those other 2 issues instead.

In the meantime, I'm pushing what I currently have on my local in a PR that people can play with (not ready for review - has some cruft in it).