Fix remote repository connection checking

[chrismwendt]

Previously, the frontend would neglect to check that the repository is associated with a GitHub external service in addition to checking that the repository hostname is github.com before finding an access token and looking up the repository on GitHub.com via HTTPS.

This adds a check to make sure the repository is associated with the external service before using the token from the external service.

• @nicksnyder Does this look like an appropriate change to make? You probably have a better understanding of the data model and relationships between repositories, URLs, access tokens, and external services than I do.
• @slimsag Does this actually fix the problem you saw in the issue below?

Fixes #1856

[codecov-io]

Codecov Report

Merging #2102 into master will decrease coverage by <.01%.
The diff coverage is 35.29%.

Impacted Files	Coverage Δ
cmd/frontend/backend/repos_mock.go	0% <ø> (ø)	⬆️
cmd/frontend/backend/repos_vcs.go	36.11% <27.27%> (-1.15%)	⬇️
cmd/frontend/backend/repos.go	28.71% <50%> (ø)	⬆️

[nicksnyder]

Your explanation makes sense and this propose change generally looks correct to me.

[tsenart]

Thanks for helping with this!

[chrismwendt]

Thanks for the suggestions and review 🙇