Allow to reference board in ancestor group when creating epic

What does this MR do?

When an epic is created from an epic board, user can can select a group where the epic will be created. This group can be either the same group as board's group or any subgroup.

For this reason when we are searching for the board, it can exist in any ancestor group.

Related to #336645 (closed)

DB queries:

• query to find board by id in the group hierarchy: https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/5942/commands/20105
• query to find all boards in the group hierarchy (currently not used, because we pass specific board id to the finder, but mentioning it here anyway because finder could be called in future w/o board_id on other place): https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/5942/commands/20108

Screenshots or Screencasts (strongly suggested)

How to setup and validate locally (strongly suggested)

Sample graphql query:

mutation {
  boardEpicCreate(input: {boardId: "gid://gitlab/Boards::EpicBoard/1", groupPath: "h5bp/sgroup1", listId: "gid://gitlab/Boards::EpicList/1", title: "test3"}) {
    epic {
      id
      title
    }
    errors
  }
}

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