Fix #869: Allow logged out user to view content on default instance

[uiri]

This is at the works-on-my-server stage. It still needs tests.

Open questions:

• Is a variable buried deep inside the source the right place for configuring the default instance?
• Should support for no default instance be maintained? If not, what should the default default instance be?

For now, I've left the default default instance as mastodon.social even though obviously I use a different value for my testing.

[nolanlawson]

This is an interesting PR, thanks! I'm not sure I'd like to merge it, though, since I kind of disagree that Pinafore should privilege any one instance over another one (e.g. mastodon.social). I think it should be a neutral third-party app (unless maybe when self-hosted by another instance, TBD).

If you want to override currentInstance then unfortunately it's going to be a bit hard because it's referenced everywhere. The best solution may be to turn it into a computed property (e.g. here and then make sure that currentInstance is never set() but rather computed from some other property. Otherwise you will probably miss a spot.

[nolanlawson]

Closing for now, as I'm not sure about having a default instance for all of Pinafore. Maybe in order to solve #35, we could instead have a global environment variable that could be used during building, and which would auto-log in to the given instance by default using a cookie.