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
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.
diff --git a/templates/manifest.gjson b/templates/manifest.gjson
index e1d3d0d..8d93b2e 100644
@@ -52,7 +52,7 @@