Add cherry-pick action

When cherry pick was successful, open new created change. To implement
the URL computation, existing method _computeChangePath() was moved
from change view to REST client behaviour to reuse from change action
element.

TODOs:
- implement branch name suggestion oracle
- improve error handling when branch or message is missing (alert)
- improve error handling when conflict occurs (409)
- improve error handling when branch doesn't exist (400)

Test Plan:

1. Open a change
2. Click Cherry Pick button
3. Type destination branch and click confirm button
4. Confirm, that the change cherry-picked and new change is opened

Bug: Issue 3906
Change-Id: I587f541dbc3338dc138f306c400f5f8857fd16a3
diff --git a/polygerrit-ui/app/behaviors/rest-client-behavior.html b/polygerrit-ui/app/behaviors/rest-client-behavior.html
index 55cd2b1..f6897cd 100644
--- a/polygerrit-ui/app/behaviors/rest-client-behavior.html
+++ b/polygerrit-ui/app/behaviors/rest-client-behavior.html
@@ -98,6 +98,10 @@
       }
       return v;
     },
+
+    changePath: function(changeNum) {
+      return '/c/' + changeNum;
+    },
   };
 
   window.Gerrit = window.Gerrit || {};