Handle repository names consistently in integrations

Created by: sqs

Our integrations (browser extension (in some cases) and editor extensions (everywhere)) make the assumption that they can derive the repository name from the repository's clone URL. This often works, but it breaks if the site config contains repos.list with non-conventional repo names or any code host configs with non-default repositoryPathPattern.

One obvious instance of this incorrect assumption is: "if the repository's name starts with github.com/, it's a GitHub repository". Many sites omit the github.com/ prefix from their repos.

This leads to problems with the editor extensions where they do not know how to open the current file on Sourcegraph.

We should spec this out and make sure all integrations handle this correctly.