roomList: Show indicator while channel is ensured

After connecting to the network, rooms often take some time before
they are joined. Show a "content-loading" symbol next to each room
if the room takes more than 3 seconds to ensure.
1 job for wip/fmuellner/room-progress in 2 minutes and 48 seconds (queued for 3 seconds)
Status Name Job ID Coverage
passed build-polari #1788