Warn before deleting confluence-administrators or confluence-users group

These groups have some special functionality, and administrators might not realise that they're special. It would be good to pop up a warning of some kind before letting them be removed.

In particular, these groups do the following:

• confluence-users: all users created through the admin screen or public signup are added to this group by default; when creating a space, granting permisions to 'logged-in users' means this group
• confluence-administrators: all users in this group have permission checks skipped, allowing them to see otherwise inaccessible spaces and "rescue" spaces that have become inaccessible to normal users.

Note that Confluence can still be administered and operated normally if these groups are removed. The warning is purely to stop people removing these groups unless they understand what they're doing.