Change app and api to support organization id on topics

What does this MR do and why?

Follow-up from !163410 (merged).

It changes:

• Database
• • Add a new Index to support non_private_projects_count query using organization id.
• App
• • Alters http://gdk.test:3000/explore/projects/topics to query topics using organization_id.
• • Alters topics merge service to validate if two topics being merged belongs to the same organization
• • Use organization id in topics finder
• API
• • Alters Topics API to use organization to search for topics
• • Updates docs

Notes

1. As all topics in production belong to the default organization, these changes will not affect the current behavior.

2. There are still some changes to add to:

• GraphQL
• https://gitlab.com/gitlab-org/gitlab/-/blob/da253cdf19dfd7da39cbb439382f342e07d0023c/app/models/project.rb#L836
• https://gitlab.com/gitlab-org/gitlab/-/blob/da253cdf19dfd7da39cbb439382f342e07d0023c/app/models/project.rb#L3433

These will be added in a follow-up

Issue #463254 (closed)