Refactor: Use a global AbortController

[marcelklehr]

The rationale for this is that request code currently lives in components without any way to keep the code DRY.
Ideally, request code would live in vuex actions IMO, but then we lose the ability to cancel requests from views.
This PR removes logic from views that handles request cancellation and moves it to the router, which cancels requests upon route changes. I've also introduced a global priority queue for requests to replace the semaphore business, which also would have to be added to every request otherwise.

[artonge]

Overall, I think only requests that fetch photos or albums should be canceled. They are read only requests and the only ones that can take a long time to load. The other ones are here to alter the DB, so they should not be canceled.

I would prefer for the views to keep the responsibility to cancel the requests, and not a central service. Using a mixin here might make more sense.

[marcelklehr]

Using a mixin here might make more sense.

Mh, but then wouldn't we need a mixin and a cancelRequest property for every request we do? Because every request gets its own cancelRequest property to cancel on view destruction. It's a lot of manual book keeping.

Update: Or do you mean putting only the cancelAll() call into a mixin, so views can decide whether they want to use it?

[artonge]

/compile amend /

[marcelklehr]

/compile amend /