Loads up the admin registration center.

• Accessed at index.php?action=regcenter
• Requires the moderate_forum permission
• Uses the Register template and Login language file
• Sets the adminIndex to registration_center
• Sets up all available sub-actions
• Displays sections for activating/approving members based on settings and whether or not any members need activating or approving
• Calls the appropriate function based on the sub-action
• If no sub-action is set, calls AdminBrowse(), unless registration is disabled (in which case it calls AdminRegister())