Strip HTML and/or truncate excessively long comments on vulnerability_feedback table

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR introduces a batched background migration job that strips HTML tags and/or truncates excessively long comments on vulnerability_feedback table

Related to #383703 (closed) step 4

Database review

Truth be told this is going to be a no-op on GitLab.com because our longest record in that table is 2_817 characters long but @ahegyi was right to suggest that we don't know that's the case for our self-hosted customers. I think 50 000 is unlikely but it's better to be safe than sorry.

Batch selection

SELECT "vulnerability_feedback"."id" FROM "vulnerability_feedback" WHERE "vulnerability_feedback"."id" BETWEEN 1 AND 596672 AND (char_length(comment) > 50000) ORDER BY "vulnerability_feedback"."id" ASC LIMIT 1

https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/13624/commands/47800

SELECT "vulnerability_feedback"."id" FROM "vulnerability_feedback" WHERE "vulnerability_feedback"."id" BETWEEN 1 AND 596672 AND (char_length(comment) > 50000) AND "vulnerability_feedback"."id" >= 11 ORDER BY "vulnerability_feedback"."id" ASC LIMIT 1 OFFSET 250

https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/13624/commands/47803

SELECT "vulnerability_feedback".* FROM "vulnerability_feedback" WHERE "vulnerability_feedback"."id" BETWEEN 472478 AND 472728 AND (char_length(comment) > 50000) AND "vulnerability_feedback"."id" >= 472478

https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/13624/commands/47804

Record update

explain UPDATE "vulnerability_feedback" SET "updated_at" = '2022-12-01 18:25:43.551210', "comment" = 'definitely shorter' WHERE "vulnerability_feedback"."id" = 12

https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/13624/commands/47805

MR acceptance checklist

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