CRM-13780 Don't allow users to attempt to merge the Default Organization into another organization

Currently if a user attempts to merge the default organization with another organization it 'appears' to work but the default organization is not actually deleted. It's good that we don't delete it, but confusing for the user.

If the Default Organization is selected as the duplicate contact in the Merge screen - display an error status message (form rule error?) when Merge button is clicked and do NOT continue.

"The default organization contact can not be merged into another contact record. It is associated with the CiviCRM installation for this domain and contains information used for system functions. If you want to merge these records, you can click the link to "Flip between the original and duplicate contacts" - and then continue with the merge."

NOTE: Make the "Flip between .." text in the pop-up a clickable link which does the 'flip'.