Add 'batch lock' feature

This change adds a 'batch lock' option which, when enabled, makes the
extension display a lock button in the thread list toolbar in the
Community Console.

When this button is clicked, the user is prompted whether they want to
lock or unlock the selected messages.

After the user makes their choice, the action is performed in all the
selected threads and any error while performing it is shown to the user.

Fixes: #24

Change-Id: I70bdc698a8d4694b2f11561fdb0a0d5c17f4d3b5
diff --git a/templates/manifest.gjson b/templates/manifest.gjson
index e1d3d0d..8d93b2e 100644
--- a/templates/manifest.gjson
+++ b/templates/manifest.gjson
@@ -52,7 +52,7 @@
-    "injections/ccdarktheme_switch.css"
+    "injections/batchlock_inject.js"
   "browser_action": {},
 #if defined(CHROMIUM)