Mirror Syncing fails with Github if archived repos exist

Mirror Syncing fails with a 403 Forbidden error, with Github if archived repos exist.

This is because the archived repo is read-only and so the error occurs when trying to write a web-hook to the repo as part of the sync setup.

Following investigation, solution appears to be to automatically ignore archived Github repos when configuring with web-hook triggers. Configuring with polling appears to be unaffected, but as an additional feature we could make ignoring archived Github repos configurable for polling.

However, there are also implications to consider around previously configured mirror syncing. For example, if you later archive a web-hook triggered Github repo and then try to edit/remove its configuration, it will currently fail with a 403 error.