Use icon to indicate extension being inactive

[ThomasGreiner]

Background There have been reports of users being logged out of their account and not noticing that no flattrs have been created and submitted to flattr.com. While we're still investigating the potential causes of such an issue (incl. access token got invalidated), we can at least make it more obvious to users that they are no longer authenticated.

What to change

• [x] Use "error" state for icon whenever extension is inactive due to insufficient account data.
• [ ] Add information about account state changes to collected data.