Allow instances to be pulled from other repositories

[Galaucus]

I and a team of others are working on https://github.com/Endless-Sky-Pirate-Campaign/endless-sky

Being able to manage installs of this branch using ESLauncher2 would smooth over our work greatly. I believe MCO mentioned something about needing to change how artifacts are handled to make this work.

[MCOfficer]

What's your expectation as to how this should be configured? like, how would a person tell the launcher to use an alternative repo?

Being able to manage installs of this branch using ESLauncher2 would smooth over our work greatly. I believe MCO mentioned something about needing to change how artifacts are handled to make this work.

Yes, the artifact unblocker currently has the ES repo hardcoded. refs #140

[Galaucus]

Currently, when installing a new instance, you can pull from PR# or continuous. If another option were added below allowing the user to supply a URL to a valid ES repository to pull from that might do the trick.

[samrocketman]

I could open a pull request for this designed like #272. Would this be acceptable @Galaucus @MCOfficer ?

I could also add an option for user to provide their own personal access token.

[Galaucus]

No objections here.

[Zitchas]

This would be very useful to have, and it'd render the need to do things like creating a PR to the main repository just so that ESL2 can pick it up unnecessary.

[Crystal-RainSlide]

Another option would be just: allow install from a downloaded Endless Sky release file / downloaded plugin.

Then one can just source their instance from literally any place they want.

Also this enables offline installs.