Include option for a URL alias in the browser extensions

Created by: christinelovett

Feature request description

Provide option to leverage a URL alias in the browser extension, so that the extension can use a customer source-of-truth URL for a given repository.

Is your feature request related to a problem? If so, please describe.

Customer has a primary code host instance and a mirror of the instance. The primary instance is needed in Sourcegraph, but customer wants to prompt URL redirects to the mirror instance to protect the primary instance's performance when using the browser extension.

Additional context

Customer: https://github.com/sourcegraph/accounts/issues/246