Add migration for Boards::EpicIssueBoardPosition

What does this MR do and why?

Add Boards::EpicIsueBoardPosition model, spec factory and associations. This allows us to specify positioning for epics in Issue boards independently.

DB migration:

main: == 20230222122321 CreateEpicIssueBoardPosition: migrating =====================
main: -- create_table(:boards_epic_issue_board_positions)
main:    -> 0.0302s
main: == 20230222122321 CreateEpicIssueBoardPosition: migrated (0.3193s) ============

main: == 20230222124300 AddEpicForeignKeyToEpicIssueBoardPosition: migrating ========
main: -- transaction_open?()
main:    -> 0.0000s
main: -- transaction_open?()
main:    -> 0.0000s
main: -- execute("ALTER TABLE boards_epic_issue_board_positions\nADD CONSTRAINT fk_d8ee9d8116\nFOREIGN KEY (epic_id)\nREFERENCES epics (id)\n\nON DELETE CASCADE\nNOT VALID;\n")
main:    -> 0.0075s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0008s
main: -- execute("ALTER TABLE boards_epic_issue_board_positions VALIDATE CONSTRAINT fk_d8ee9d8116;")
main:    -> 0.0142s
main: -- execute("RESET statement_timeout")
main:    -> 0.0009s
main: == 20230222124300 AddEpicForeignKeyToEpicIssueBoardPosition: migrated (0.2197s)

main: == 20230222130510 AddEpicBoardForeignKeyToEpicIssueBoardPosition: migrating ===
main: -- transaction_open?()
main:    -> 0.0004s
main: -- transaction_open?()
main:    -> 0.0000s
main: -- execute("ALTER TABLE boards_epic_issue_board_positions\nADD CONSTRAINT fk_e273f734f4\nFOREIGN KEY (epic_board_id)\nREFERENCES boards_epic_boards (id)\n\nON DELETE CASCADE\nNOT VALID;\n")
main:    -> 0.0153s
main: -- execute("ALTER TABLE boards_epic_issue_board_positions VALIDATE CONSTRAINT fk_e273f734f4;")
main:    -> 0.0183s
main: == 20230222130510 AddEpicBoardForeignKeyToEpicIssueBoardPosition: migrated (0.1731s)

Related to #391931

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.