Add organization id to topics

What does this MR do and why?

This is the first MR that adds the sharding key (organization_id) to topics.

• It adds organization_id to the topics table and creates all related foreign keys and indexes.

Step	MR
1	Add organization id to topics - this MR
2	Change topics application code to use organization id - TBD
3	Copy project topics to organization - TBD
3	Remove the organization_id column default - TBD

Notes:

• Topics will be assigned to a default organization for now: organization 1;
• The default organization will be removed from topics, once we change all necessary code;
• As we can create topics for different organizations, they all shall remain under the same default organization for now;
• In production, there are 3 projects not associated with default org and none of them have topics:

gitlabhq_dblab=# SELECT id FROM "projects" WHERE "projects"."organization_id" != 1;
id
----------
 59701951
 61040785
 60992049
(3 rows)

gitlabhq_dblab=# SELECT projects.name project, organizations.name organization FROM projects JOIN organizations ON organizations.id = projects.organization_id WHERE projects.id IN (60992049, 59701951, 61040785);
     project     |        organization
-----------------+-----------------------------
 Test Project    | jl-test-organization
 lc-test-project | lc-test-org
 baz             | Peter Hegman's Organization
(3 rows)

gitlabhq_dblab=# SELECT * FROM project_topics WHERE project_id IN (60992049, 59701951, 61040785);
 id | project_id | topic_id | created_at | updated_at
----+------------+----------+------------+------------
(0 rows)

How to test

1. On master, add some topics:

Project.all.each do |project|
  project.topic_list = project.path
  project.save!
end

%w[Ruby Postgres JavaScript Rails Bash].each do |topic|
  Projects::Topic.create!(name: topic, title: topic)
end

2. Checkout 463254/1-add-organization-id-to-topics and migrate the database

3. On console, try to create some topics:

# Uses default organization 1
Projects::Topic.create!(name: 'topic-1', title: 'topic-1')

# Same topic can be created with a different organization
Projects::Topic.create!(name: 'topic-1', title: 'topic-1', organization_id: Organizations::Organization.where.not(id: 1).last.id)

# When topics are created through projects, they will be created in the same organization as the project
project = Project.where.not(organization_id: 1).last
topic_name = "TopicFromOrganization-#{project.organization_id}"
project.topic_list = topic_name
project.save!

Projects::Topic.for_organization(project.organization_id).where(name: topic_name)
# => [<Projects::Topic:0x000000017d065c88 id: 19, name: "TopicFromOrganization-1019", ... organization_id: 1019>]

4. Create/Edit topics trough admin page: http://gdk.test:3000/admin/topics
5. Explore created topics and projects at http://gdk.test:3000/explore/projects/topics/

#463254