DEV: A general code clean-up

[CvX]

Summary:

• removed computed usage from a glimmer component
• converted the component to gjs
• converted the connector to gjs and made it into a regular glimmer component (w/ service injections and a getter instead of setupComponent)
• used session service injection instead of Session singleton wherever possible
• moved prop initialization out of a constructor
• removed a use of string-based action
• simplified showInSidebar logic
• converted the initializer to the new class-based paradigm (w/ service injections)
• added an escape hatch (isDestroying/isDestroyed check) to a later call
• add a matching removeEventListener to window.matchMedia(…).addEventListener
• used qunit-dom in tests