Include codemirror 3.13

Download and package codemirror 3.13 for inclusion in the UI.
This may eventually replace the current side-by-side viewer,
as well as offer rich online editing to make small fixes to
existing changes.

Change-Id: I483fb46d9e18164d189826eca5cb96f9a1471c7e
diff --git a/gerrit-gwtui/BUCK b/gerrit-gwtui/BUCK
index dc47d82..cf38248 100644
--- a/gerrit-gwtui/BUCK
+++ b/gerrit-gwtui/BUCK
@@ -74,6 +74,7 @@
     '//lib:gwtjsonrpc_src',
     '//lib:gwtorm',
     '//lib:jsr305',
+    '//lib/codemirror:codemirror',
     '//lib/gwt:user',
     '//lib/jgit:jgit',
   ],