Backfill missing routes for Bot Users

What does this MR do?

This migration generates routes that are missing for bot users, based on the finding from !35711 (merged)

Query

The generated query to find bot users without routes is:

SELECT namespaces.*
FROM namespaces
INNER JOIN users ON users.id = namespaces.owner_id
WHERE (type IS NULL)
  AND users.user_type IN (2, 6, 1, 3, 7)
  AND (NOT EXISTS
         (SELECT 1
          FROM routes
          WHERE source_type = 'Namespace'
          AND source_id = namespaces.id))

Explain

This takes ~100 ms to run according to database-lab.

We have 3 bot users without routes in .com database.

The whole migration could take ~200 ms for completion on production database.

Logging

Logging has been added for both successful and failing records, it will be recorded like this in log/migrations.log

{"severity":"INFO","time":"2020-07-07T07:56:21.363Z","correlation_id":"2262e69ebffb6917a5d1c728d3f7a3ac","id":6,"path":"migration-bot","name":"GitLab Migration Bot","message":"a new route was created for the namespace"}
{"severity":"INFO","time":"2020-07-07T07:56:23.018Z","correlation_id":"c6d00663d7b62657969d22314d986101","id":8,"path":"visual-review-bot","name":"GitLab Visual Review Bot","message":"route creation failed for the namespace","errors":"Path has already been taken"}

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team