Fix links to build artifacts

[gabalafou]

Fixes:

• Fixes [BUG] - getting artifacts from the ui returns not found error conda-store#995.

Supersedes:

• Ensure artifact base url has a trailing slash #442
• Ensure api url setting has trailing slash conda-store#997

This is not the first time the build artifact links have been broken. The solution in this pull request is robust to both the recent breakage and the older one.

[soapy1]

Can confirm this resolves the issue in conda-incubator/conda-store#995!

Calling out, I'm not very familiar with ts at all. Might be worth getting another reviewer?

[gabalafou]

Excellent! I was about to add instructions for how to confirm that this works, but you're already ahead of me.

As for not being familiar with TypeScript, I think it's fine. I have made the tests much more comprehensive, so I think we can have confidence that this is working. And so long as the TypeScript checker isn't complaining (and there aren't any casts to any, which there aren't), all should be well in the world.