online-accounts: Streamline the panel

Currently, the panel has a bit different design compared to others. It features an unnecessarily prominent title, no title for the list of connected accounts, a weirdly placed offline message, and an old-school manual kind of toast on account deletion.

I've moved the title to a description of the "Add an account" list, added title and description to the list of connected accounts, and temporarily removed the offline message. Right now I'm trying to add a proper toast on account deletion. Then I plan to work on adding an AdwBanner for the offline message, but that will require bumping up libadwaita dependency to 1.3.

Related to !1669 (merged)
Fixes #2201 (closed), #2192