Remove outdated relationship-editing interfaces

 * Removed all the individual relationship edit pages.
 * Removed the [ Edit | Remove ] links.
 * Removed the "Use in a relationship" links.
 * Removed the "Relate to URL" links.
 * Removed the "Relate to" popups.
Load cardinality info for link types
MBS-6234: Mini-relationship-editors initial commit

 * Added new mini relationship editors to all the /{edit,create} entity pages, except for releases.
 * Made the release relationship editor use /ws/js/edit.
 * Made the relationship and external link editors share mostly the same code.
 * Made the relationship editor JS part of the edit manifest.
 * Updated knockout to 3.1.0.
 * Changed the multiselect plugin thing I wrote a while ago (for instrument/vocal selection) to use knockout instead of pure jQuery (which saves quite a bit of code).
 * Moved/renamed/split the JS files for the relationship editor to better reflect the current structure/hierarchy of the code. Under root/static/scripts/relationship-editor:
 * release.js has the code for the release relationship editor.
 * generic.js has the code for the mini relationship editors.
 * common/* holds everything shared by the two.
 * Renamed "type" to "entityType." Throughout our JavaScript, objects that represent entities had "type" attributes that indiciated what entity type they were. But "type" was a bad name
Misc. problems spotted by chirlu
Fix the fix for the typo
Clarify button text for adding another relationship for a specific link type
Fix alignment of add-relationship buttons
Consolidate the URL error checking code into a single spot

Makes it much easier to fix an issue where forms couldn't be submitted
because of existing relationships using a deprecated link types (and
fixes that issue).
Fix typo in WS::js::Edit, add tests for medium edit/delete
Fix empty area bubble on artist edit form

currentSelection should always be an MB.entity so that it has an html
method.
Fix place performances page
Remove reference to Form::Role::ExternalLinks

It no longer exists. Form::Role::Relationships took its place.
Dim and de-cursor the pen icon if the relationship is removed
Check if a link type has children before traversing them
Fix QUnit tests
Don't overwrite source entity gid with tmp gid

Fixes an issue where you could relate an entity to itself, because it
couldn't look up the original one in the cache.
Only accept integers in the date field
Style the change-direction button
Prevent entities in a relationship from being the same in Edit::Relationship::Create->initialize
Remove duplicate <hr> in area sidebar