Allow users to list repositories currently being cloned

Created by: mrnugget

This introduced a new filter to the GraphQL: cloneStatus. If set, it returns only repositories that have the given clone status.

With that we can reduce the number of booleans we use when interacting with the API: instead of querying for {cloned: true, notCloned: false} (which we did even though the notCloned was useless) we can query for {cloneStatus: "CLONED"}.

And now we can also query for {cloneStatus: "CLONING"}.

This also changes how the site-admin UI works:

• previously: listing "not cloned" would include "cloning" repositories
• now: listing "not cloned" only lists repositories that are "not cloned".

Subpart of this ticket: https://github.com/sourcegraph/sourcegraph/issues/39328

Test plan

• unit tests
• manual testing