Change icon in notifications popup in MediaWiki's "Echo" extension

Wikimedia

The "messages" notification panel currently uses a single speech bubble icon. We should use a double speech bubble icon instead. This task involves working with the "Echo" MediaWiki extension, and the patch should be submitted to Gerrit. More details can be found on the Phabricator task: https://phabricator.wikimedia.org/T119377, questions can be asked there or on IRC.