search: no way to undo fork:no default behavior change in 3.14

Created by: slimsag

A user at https://app.hubspot.com/contacts/2762526/company/407948923 wants to go back to having forks included in search results by default (single user preference, one of their main repositories is technically a "fork" but is actually the official repository).

I also believe we have another customer who has many repositories that are forks and would be very surprised to learn there is not a way to undo this behavior change globally. https://about.sourcegraph.com/blog/sourcegraph-3.14#forks-and-archived-repositories-are-excluded-from-search-results-by-default

I think us not being able to at least undo this behavior change (ideally would be user, org, or global settings configurable) is a massive ticking time bomb for us and we should prioritize landing this ASAP. cc @christinaforney @rvantonder we should ship this in 3.15 or a patch release very soon after