MDL-39312 cache: interfaces for managing lock instances.

Bug: There was no way to create an instance of a lock plugin for use
within Moodle.
Solution: Implemented management interfaces as part of cache/admin.php
to allow for instances to be added and deleted.
This was done in along the same lines of adding store instances.

theme/bootstrapbase/less/moodle/admin.less

@@ -722,7 +722,8 @@ img.iconsmall {
 }
 
 #core-cache-rescan-definitions,
-#core-cache-mode-mappings .edit-link {
+#core-cache-mode-mappings .edit-link,
+#core-cache-lock-summary .new-instance {
     margin-top: 0.5em;
     text-align: center;
 }