Allow file annotations (blame) in side-by-side diff

Using CodeMirror's lint addon on the UI to display the blame
annotations. It works on both sides of the side-by-side
diff and also supports the auto-merge commit. It requires manual
step to enable the annotations to avoid any unnecessary git
processing and network traffic between the server and client.

Introduces a new dependency on blame-cache in gerrit-server
to reuse BlameCache.

In a following change, the gutter showing the blame info will be
made clickable. Clicking on the gutter will open a new tab that
takes the user to the corresponding change in Gerrit.

The commit SHA-1 hashes are currently not selectable. Making it
so might require an upstream change in CodeMirror's lint addon.

Bug: Issue 1642
Change-Id: I6267d30cbee448f8137e11c7120959dc424eaeeb

lib/BUCK

@@ -261,3 +261,11 @@ maven_jar(
   sha1 = 'd9a09f7732226af26bf99f19e2cffe0ae219db5b',
   license = 'DO_NOT_DISTRIBUTE',
 )
+
+maven_jar(
+  name = 'blame-cache',
+  id = 'com/google/gitiles:blame-cache:0.1-9',
+  sha1 = '51d35e6f8bbc2412265066cea9653dd758c95826',
+  license = 'Apache2.0',
+  repository = GERRIT,
+)