repo-updater: BitbucketServerUsernameMigration
Created by: tsenart
This commit implements a best effort migration that extracts the
username from the url field of Bitbucket Server external service
configurations and sets in the username field if missing or empty.
The username field became required in #3299 because it was always
actually required for things to fuction properly.
If there are configs without the username field set not the user
part of the url set, the admins will need to intervene to fix their
configs manually.
Fixes #3592