Filter changesets with no reviewer

Created by: malomarrec

Problem

The main problem users face when they are trying to get a batch change over the finish line is making sure each changeset is in the radar of the downstream repository owner. A failure mode is when there is no reviewer on the changeset. Although this can be mitigated on the codehost side, eg. by setting auto-assignment rules, there will always be repositories without the proper setup, especially in large organizations.

Currently Batch Changes surfaces the review status, but does not discrimate between "review is pending and there is a reviewer assigned" and "there is no reviewer assigned". Both are represented as "Pending"

Proposed change

Add another review status for "no reviewer assigned".

Impacted customers

• https://github.com/sourcegraph/accounts/issues/565