Display a warning when editing relationship types with data in

Follow-on from: http://forum.civicrm.org/index.php/topic,34144.0.html

Currently relationship types that already have data in can be edited. This can mess up data in the site if it is done without considering the consequences. ie: swapping the contacts used for contact A and contact B.

However we should allow the contact types to be changed, in case we are narrowing the contact type down to a new sub-type, for example.

Proposed solution:

If relationships of a certain type already exist, then when editing that relationship type the 'Contact type' inputs are hidden by default and a message says:

'Relationships of this type exist in the database. Changing the contact types now may cause issues in your site. <Edit the contact types> anyway.' Clicking 'Edit the contact types' would show the contact type fields again.

(Wording flexible!)