Add username to users mapper

When importing a group using Direct Transfer we map users on the source instance to those on the destination instance by matching email addresses. Sometimes the @username on the source instance will differ from the one on the destination instance. This poses a problem when importing issue and mr descriptions and system notes that @-mention a user, as we had no way of mapping the source username to the destination username. see #413623 (closed)

The changes here address that problem.

• updating the get_members graphql query to include the username in the returned result.
• updating the user_mapper with methods to create and save source_usernames and destination_usernames in a hash in redis.
• adding a username pipeline for updating all issue/mr/note username mentions with the correct username where necessary.

Screenshots or screen recordings

Before

After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

• sign in as a new user on gitlab.com

• create a group

• create a project in the group

• create an issue and assign it to self

• add your normal gitlab user as a member to the group

• create a user on gdk.test with the same email address, but a different @username

• impersonate your normal gitlab member on gdk.test

• import the group created above from gitlab.com using Direct Transfer

• observe that the issue author should have the correct first and last name and the @username in the activity notes should have the destination @username.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #413623 (closed)