BE changes for organization level activity page

What does this MR do and why?

As per #415300 (closed) this MR:

• adds a new activity route for organizations,
• adds new menu item under the manage
• adds empty activity page

Query Plan

Query	postgres.ai	Screenshot
SELECT * FROM "projects" WHERE "projects"."organization_id" = 1 ORDER BY "projects"."last_activity_at" DESC LIMIT 1000;	https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/26619/commands/83389
explain SELECT * FROM "namespaces" WHERE "namespaces"."organization_id" = 1 LIMIT 1000;	https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/26619/commands/83390
SELECT "events".* FROM "events" INNER JOIN ( SELECT "id" FROM "projects" WHERE "organization_id" = 1 LIMIT 1000 ) AS "project" ON "project"."id" = "events"."project_id" WHERE "events"."action" = 5;	https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/26619/commands/83400
SELECT "events".* FROM "events" INNER JOIN ( SELECT "id" FROM "projects" WHERE "organization_id" = 1 LIMIT 1000 ) AS "project" ON "project"."id" = "events"."project_id" WHERE "events"."action" = 7;	https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/26619/commands/83401
SELECT "events".* FROM "events" INNER JOIN ( SELECT "id" FROM "projects" WHERE "organization_id" = 1 LIMIT 1000 ) AS "project" ON "project"."id" = "events"."project_id" WHERE "events"."action" = 6;	https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/26619/commands/83402

Screenshots or screen recordings

How to set up and validate locally

1. Checkout the branch.
2. Access an organization with some projects/groups.
3. Visit the activity page (Organization/Activity).

note: if you don't see organizations page, you might need to enable ui_for_organizations FF.

Related to #415300 (closed)