LPS-43226 - Selected site should not be available to be selected again

Hey @natecavanaugh,

Attached is an update for http://issues.liferay.com/browse/LPS-43226.

NOTE: Notes about code changes can be found at https://github.com/Robert-Frampton/liferay-portal/commit/2d6056922094853cccb61fb83a93b17077c66165.

If everything is good for this pull, we need to apply these changes to other pages in users_admin (Roles, User Groups, etc.).  Please assign back to @Robert-Frampton when reviewed so we can make all the changes and push them all at once.

Please let me know if you have any questions.  Thanks!