Steal MigrateU2fWebauthn background migration

What does this MR do?

This MR follows up on !42159 (merged) / #232670 (closed), draining the background migration for U2F registrations introduced in !42159 (merged) and migrating any leftover U2F devices to WebAuthn.

Migration Output

Stealing Migration

== 20201026185514 EnsureU2fRegistrationsMigrated: migrating ===================
== 20201026185514 EnsureU2fRegistrationsMigrated: migrated (0.0877s) ==========

== 20201026185514 EnsureU2fRegistrationsMigrated: reverting ===================
== 20201026185514 EnsureU2fRegistrationsMigrated: reverted (0.0000s) ==========

Migrating 1000 records takes approx. 16 seconds (on my machine):

== 20201026185514 EnsureU2fRegistrationsMigrated: migrating ===================
== 20201026185514 EnsureU2fRegistrationsMigrated: migrated (16.3710s) =========

Gitlab.com has 1591 u2f registrations left, so this should take something between 20 and 40 seconds, depending on the system environment.

Increase text limit

== 20201117075742 ChangeWebauthnXidLength: migrating ==========================
-- transaction_open?()
   -> 0.0000s
-- current_schema()
   -> 0.0004s
-- execute("ALTER TABLE webauthn_registrations\nADD CONSTRAINT check_e54008d9ce\nCHECK ( char_length(credential_xid) <= 340 )\nNOT VALID;\n")
   -> 0.0012s
-- current_schema()
   -> 0.0004s
-- execute("SET statement_timeout TO 0")
   -> 0.0003s
-- execute("ALTER TABLE webauthn_registrations VALIDATE CONSTRAINT check_e54008d9ce;")
   -> 0.0050s
-- execute("RESET ALL")
   -> 0.0005s
-- execute("ALTER TABLE webauthn_registrations\nDROP CONSTRAINT IF EXISTS check_242f0cc65c\n")
   -> 0.0009s
== 20201117075742 ChangeWebauthnXidLength: migrated (0.0531s) =================

== 20201117075742 ChangeWebauthnXidLength: reverting ==========================
== 20201117075742 ChangeWebauthnXidLength: reverted (0.0000s) =================

SQL Queries

Find start id in batch:

SELECT
    "u2f_registrations"."id"
FROM
    "u2f_registrations"
    LEFT JOIN webauthn_registrations ON webauthn_registrations.u2f_registration_id = u2f_registrations.id
WHERE (webauthn_registrations.u2f_registration_id IS NULL)
ORDER BY
    "u2f_registrations"."id" ASC
LIMIT 1

https://postgres.ai/console/shared/0daca4e3-5fd5-4a53-8676-648932f237ca

Find end id in batch:

SELECT
    "u2f_registrations"."id"
FROM
    "u2f_registrations"
    LEFT JOIN webauthn_registrations ON webauthn_registrations.u2f_registration_id = u2f_registrations.id
WHERE (webauthn_registrations.u2f_registration_id IS NULL)
    AND "u2f_registrations"."id" >= 131404
ORDER BY
    "u2f_registrations"."id" ASC
LIMIT 1 OFFSET 100

https://postgres.ai/console/shared/145aa6ee-dc1c-4bb1-a7ec-aae9fc9a099e

Load all records in the batch:

SELECT
    "u2f_registrations".*
FROM
    "u2f_registrations"
    LEFT JOIN webauthn_registrations ON webauthn_registrations.u2f_registration_id = u2f_registrations.id
WHERE (webauthn_registrations.u2f_registration_id IS NULL)
    AND "u2f_registrations"."id" >= 131404
    AND "u2f_registrations"."id" < 131526

https://postghttps://postgres.ai/console/shared/b55cee23-3dbc-4b2a-87be-998036b65a8b

Does this MR meet the acceptance criteria?

Conformity

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

Availability and Testing

• X ] 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

Related to #232670 (closed)