Improve duplicate membership warning

First, rewrote the warning when adding a new membership for someone who has an existing membership.

• the warning shows up only when they select a membership organization for which they have a membership
• the wording was clumsy
• it needed a title

Second, if you go to Add Membership (either through the quick button or the menu) and pick a contact, there wasn't a check to see if the contact has a membership. I've added that in, looking up the contact's memberships when they're selected and setting a warning if the member organization of the existing membership matches the one selected.

https://github.com/civicrm/civicrm-core/pull/559