Deleting XWikiAllGroup loses the default/implicit rights

Steps:

• Go to Administration - Groups and delete the XWikiAllGroup group

Effects:

• The users lose their rights. They have just the default ones (view, register, etc.)
• When registering new users, the XWikiAllGroup is recreated, but it has no rights set.
• In the case the #allgroupimplicit# is set, the XWikiAllGroup is not even recreated in the Admin Groups UI. Also the Rights don't preserve the XWikiAllGroup entry (even if it should be implicit) and the rights again are reseted.
• If XWikiAllGroup is restored from recycle bin, again the rights are lost, see also XWIKI-11624

Possible solution: Disable the ability to delete the XWikiAllGroup from Administration (at least, page level could be disabled too).
Just like we have in Administration - Users the delete current user disabled, we could use the same solution (#grayed# param in livetable from #getusers.vm#) to disable the deletion of this default group.

We should consider this group a default / vital one and protect it from deletion.
In the case the #initialGroups# is changed from #xwiki.cfg# we could protect on deletion all the groups listed.

Note: The affects version is estimated, since I couldn't identify when exactly the functionality was added.

Somewhat related to XWIKI-15394