Add ApprovalRule model

What does this MR do?

See https://gitlab.com/gitlab-org/gitlab-ee/issues/8647 for overall description.

Add several new tables to support #1979 (closed)

• approval_merge_request_rules
• approval_project_rules
• approval_merge_request_rules_users (HABTM to User)
• approval_merge_request_rules_groups (HABTM to Group)
• approval_project_rules_users (HABTM to User)
• approval_project_rules_groups (HABTM to Group)
• approval_merge_request_rules_approvals (HABTM for persisting which approval rules was satisfied by an approval)
• approval_merge_request_rules_sources (record MR rule's project rule source)

The associations are:

• MergeRequest has_many ApprovalMergeRequestRule
• Project has many ApprovalProjectRule
• ApprovalMergeRequestRule has_and_belongs_to_many User
• ApprovalMergeRequestRule has_and_belongs_to_many Group
• ApprovalMergeRequestRule has_one ApprovalProjectRule
• ApprovalProjectRule has_and_belongs_to_many User
• ApprovalProjectRule has_and_belongs_to_many Group
• ApprovalMergeRequestRule has_and_belongs_to_many Approval

Query samples

#approvers

SELECT "users".* FROM (
  SELECT "users".* FROM "users"
  INNER JOIN "approval_merge_request_rules_users"
  ON "users"."id" = "approval_merge_request_rules_users"."user_id"
  WHERE "approval_merge_request_rules_users"."approval_merge_request_rule_id" = 1

  UNION

  SELECT "users".* FROM "users" INNER JOIN "members"
  ON "members"."user_id" = "users"."id" AND "members"."type" IN ('GroupMember') AND "members"."source_type" = 'Namespace' AND "members"."requested_at" IS NULL
  WHERE "members"."source_id" IN (
    SELECT "namespaces"."id" FROM "namespaces"
    INNER JOIN "approval_merge_request_rules_groups"
    ON "namespaces"."id" = "approval_merge_request_rules_groups"."group_id"
    WHERE "namespaces"."type" IN ('Group') AND "approval_merge_request_rules_groups"."approval_merge_request_rule_id" = 1
  )
) users WHERE ("users"."id" != 1)

What are the relevant issue numbers?

#1979 (closed) #8647 (closed)

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?