Reset Github contributors list when loading data

[pgengler]

The Github contributors list for an addon seems to be append-only; contributors are never removed. This is a problem when an addon specifies an invalid repo URL at some point, because then we pick up the contributors from the other repo; then, when the addon updates to use the correct URL we never clear the list and so the addon's actual contributors are appended to the ones from the other repo, rather than replacing them.