Add data migration to remove duplicate DastSiteTokens with same token

What does this MR do?

This merge request adds a data migration to remove the existing duplicates of DastSiteToken with the same token.

The data migration is a regular migration because:

• this is a low traffic feature with 371 rows in the dast_site_tokens table on gitlab.com.
• it should be fast. It currently runs in 1.2 s
• To make sure that RemoveDuplicateDastSiteTokens has run before we run the follow-up MR. The follow-up merge request to this one will add a unique index.

This change is part of the following tasks required for issue 338191

add a data migration to remove existing duplicates

Raw SQL to find the DastSiteToken with duplications:

WITH duplicate_tokens AS (
    SELECT
        id,
        rank() OVER (PARTITION BY token ORDER BY id) r
    FROM dast_site_tokens)
DELETE FROM dast_site_tokens c USING duplicate_tokens t
WHERE c.id = t.id
    AND t.r > 1;

Time: 9.648 ms

Query Plan

Migration execution time = 1.6 s

Migration 20210825182303 up

== 20210825182303 RemoveDuplicateDastSiteTokensWithSameToken: migrating =======
-- execute("WITH duplicate_tokens AS(\n                  SELECT id, rank() OVER (PARTITION BY token ORDER BY id) r FROM dast_site_tokens\n                )\n                DELETE FROM dast_site_tokens c USING duplicate_tokens t\n                WHERE c.id = t.id AND t.r > 1;")
   -> 0.0054s
-- add_index(:dast_site_tokens, :token, {:name=>"index_dast_site_token_on_token", :unique=>true})
   -> 0.0026s
== 20210825182303 RemoveDuplicateDastSiteTokensWithSameToken: migrated (0.0082s) 

Migration 20210825182303 down

== 20210825182303 RemoveDuplicateDastSiteTokensWithSameToken: reverting =======
-- remove_index(:dast_site_tokens, :token, {:name=>"index_dast_site_token_on_token"})
   -> 0.0084s
== 20210825182303 RemoveDuplicateDastSiteTokensWithSameToken: reverted (0.0085s) 

Screenshots or Screencasts (strongly suggested)

How to setup and validate locally (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• 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