Adds OR filtering by author for epics

What does this MR do and why?

Allows using OR filtering on epic's authors. This is similar to previously added OR filtering by labels (!110643 (merged))

Related to #382969 (closed)

DB queries

• listing epics: https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/15147/commands/52617

SELECT "epics".*
FROM "epics"
WHERE "epics"."group_id" IN
    (SELECT "namespaces"."id"
     FROM "namespaces"
     WHERE "namespaces"."type" = 'Group'
       AND (traversal_ids @> ('{9970}')))
  AND "epics"."author_id" IN
    (SELECT "users"."id"
     FROM "users"
     WHERE (LOWER("users"."username") IN (LOWER('jprovaznik'),
                                          LOWER('janp'))))
ORDER BY "epics"."id" DESC
LIMIT 101

• listing epics on epic board: https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/15147/commands/52618

SELECT "epics".*
FROM "epics"
LEFT OUTER JOIN "boards_epic_board_positions" ON "epics"."id" = "boards_epic_board_positions"."epic_id"
AND "boards_epic_board_positions"."epic_board_id" = 1
WHERE "epics"."group_id" IN
    (SELECT "namespaces"."id"
     FROM "namespaces"
     WHERE "namespaces"."type" = 'Group'
       AND (traversal_ids @> ('{9970}')))
  AND "epics"."author_id" IN
    (SELECT "users"."id"
     FROM "users"
     WHERE (LOWER("users"."username") IN (LOWER('jprovaznik'),
                                          LOWER('janp'))))
  AND "epics"."state_id" = 1
  AND (EXISTS
         (SELECT "label_links".*
          FROM "label_links"
          WHERE "label_links"."target_type" = 'Epic'
            AND ("epics"."id" = "label_links"."target_id")
            AND "label_links"."label_id" = 43
          LIMIT 1))
ORDER BY "boards_epic_board_positions"."relative_position" ASC NULLS LAST,
                                                               epics.id DESC
LIMIT 100
OFFSET 0

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

1. Enable the invite modal

   Feature.enable(:or_issuable_queries)

2. Sample query:

• For epic listing:

query {
  group(fullPath: "flightjs") {
    id
    epics(or: {authorUsername: ["valerie", "trina"]}) {
      nodes { 
        id
        author {
          username
        }
      }
    }
  }
}

• For epic boards:

query {
  group(fullPath: "flightjs") {
    id
    epicBoards {
      nodes {
        id
        lists {
          nodes {
            id
            epics(filters: {or: {authorUsername: ["valerie", "trina"]}}) {
              nodes {
                id
                author {
                  username
                }
              }
            }
          }
        }
      }
    }
  }
}

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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