Fix "offline is very buggy"

[niol]

As offline was (when I worked on it in 2019) and still is working great for me (although I did not yet make the effort to update to the latest code), It would be great to document here what is wrong with it. The code has undergone many refactoring tasks since I played with it.

[desbest]

Does anyone want to volunteer for bug testing?

[jtojnar]

I think I have mostly seen random issues like:

• Service worker would report that the site has updated every time.
• Sometimes, after log in, articles would not start downloading until I reloaded the page.
• Logging out while syncing was in progress would complain loudly.
• Log out (selfoss.db.clear) would not work after database failure. Fixed by 45c7e9e.

And then rarely some promise failures that would disable the offline mode.

I think the way forward is rewriting the model layer mostly from scratch, as it is currently too tightly coupled with the app state, the view and, most annoyingly, itself.