Epic boards epic list

What does this MR do?

DB migration

$ bundle exec rake db:migrate
== 20210104163218 AddEpicBoardPositionIndex: migrating ========================
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:boards_epic_board_positions, [:epic_board_id, :epic_id, :relative_position], {:name=>"index_boards_epic_board_positions_on_scoped_relative_position", :algorithm=>:concurrently})
   -> 0.0016s
-- add_index(:boards_epic_board_positions, [:epic_board_id, :epic_id, :relative_position], {:name=>"index_boards_epic_board_positions_on_scoped_relative_position", :algorithm=>:concurrently})
   -> 0.0195s
== 20210104163218 AddEpicBoardPositionIndex: migrated (0.0213s) ===============

$ bundle exec rake db:rollback
== 20210104163218 AddEpicBoardPositionIndex: reverting ========================
-- transaction_open?()
   -> 0.0000s
-- indexes(:boards_epic_board_positions)
   -> 0.0029s
-- remove_index(:boards_epic_board_positions, {:algorithm=>:concurrently, :name=>"index_boards_epic_board_positions_on_scoped_relative_position"})
   -> 0.0051s
== 20210104163218 AddEpicBoardPositionIndex: reverted (0.0083s) ===============

Query plan

SQL query to get epics for an epic board lists ordered by the board's position (with null position last). Query plan is from a local deployment because we don't have epic board records yet: https://explain.depesz.com/s/xBx8

SELECT "epics".*
FROM "epics"
LEFT OUTER JOIN "boards_epic_board_positions" ON "boards_epic_board_positions"."epic_id" = "epics"."id"
WHERE "epics"."group_id" IN
    (WITH RECURSIVE "base_and_descendants" AS (
                                                 (SELECT "namespaces".*
                                                  FROM "namespaces"
                                                  WHERE "namespaces"."type" = 'Group'
                                                    AND "namespaces"."id" = 28)
                                               UNION
                                                 (SELECT "namespaces".*
                                                  FROM "namespaces",
                                                       "base_and_descendants"
                                                  WHERE "namespaces"."type" = 'Group'
                                                    AND "namespaces"."parent_id" = "base_and_descendants"."id")) SELECT "namespaces"."id"
     FROM "base_and_descendants" AS "namespaces")
  AND "epics"."state_id" = 1
  AND (EXISTS
         (SELECT "label_links".*
          FROM "label_links"
          WHERE (label_links.target_type = 'Epic'
                 AND label_links.target_id = epics.id)
            AND "label_links"."label_id" = 63
          LIMIT 1))
  AND ("boards_epic_board_positions"."epic_board_id" = 1
       OR "boards_epic_board_positions"."epic_board_id" IS NULL)
ORDER BY boards_epic_board_positions.relative_position ASC NULLS LAST,
                                                           epics.id DESC
LIMIT 100;

Architecture solution question

It was discussed on the related issue why we don't reuse the existing board/list issue's API/model. This is a valid question and I have to admit it's very tempting to reuse it because adding extra tables/models/services adds additional overhead. On the other side I think having both separated is the clean approach (details in the linked discussion). I'm open to refactor this if others feedback is we should reuse existing models/tables/services.

Related to #233436 (closed)

Screenshots (strongly suggested)

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