ENHANCEMENT: Moderator UI using built-in GridField

Less reliance on 3rd party modules, this replaces the TagField dependancy with the SilverStripe native GridField. 

Allows for partial search on names, email and nickname to find Member to add. Further, only allows link and unlink of Member in this many_many relationship.

Displays useful data columns for site admins to see who is a moderator, their contact email and last visit date for moderator reviewing purposes.