Refactor content scripts/injections bridge

In order to use the existent bridge with more features other than the
profile indicator, this change refactors the bridge code so the content
script event listener can handle requests being done via different
prefixes.

Change-Id: I7c8e7e62068b7326a2eeec18d871a44114d6c259
diff --git a/templates/manifest.gjson b/templates/manifest.gjson
index f6b8d5f..e1d3d0d 100644
--- a/templates/manifest.gjson
+++ b/templates/manifest.gjson
@@ -34,7 +34,7 @@
     {
       "matches": ["https://support.google.com/s/community*", "https://support.google.com/*/thread/*"],
       "exclude_matches": ["https://support.google.com/*/thread/new*"],
-      "js": ["common/content_scripts.js", "content_scripts/profileindicator_inject.js"]
+      "js": ["common/content_scripts.js", "common/cs_event_listener.js", "content_scripts/profileindicator_inject.js"]
     },
     {
       "matches": ["https://support.google.com/*/profile/*"],