-Improved group loading from the database:
--TShock will now attempt to load any available valid group data even if there are invalid records and will also report proper errors / warnings.
--"superadmin" is now a reserved group name.
--Groups with parents causing infinite parent loops, invaild parents, or parenting theirselfes will have their parent group reset.
--Double definitions of groups with the same name are no longer allowed.
--Group instances are now tried to be preserved instead of being recreated during a reload of the group data.

-Added command "/group parent" to get and set the parent of groups.
-REST Endpoint "/v2/groups/create" will no longer allow creating groups having theirselfes as parent.
-REST Endpoint "/v2/groups/update" will no longer allow setting a group's parent to theirself or setting a parent group resulting in an infinite parent loop.
-This commit should fix #482.
Merge pull request #508 from CoderCow/patch-2

Group Improvements
Closes #482