Only load ext.echo.ui if the user clicks the echo badge
There is no need to load the entire of Echo's ui module (especially
since that includes ooui widgets and their styles) on every page load.
There's only need to load the entire module if and when a user clicks
the Echo notification badge.
Also, make the echo.dm model accept an external fetchNotifications
promise so we can send the API request alongside loading the echo UI
and "feed" it into the DM for processing.
CSS adjusted to make the "jump" between the nojs and the js buttons
seem less jumpy.