Group Migration MVC: Add GMAU usage ping

🌳 What does this MR do?

Addresses: #249169 (closed)

We want to be able to report of the count of unique users that are triggering imports using the new Group Migration tool (&4374 (closed)).

This MR adds a new field (usage_activity_by_state.manage.bulk_imports.gitlab) to the usage data which can ben used to track this.

These events have been added to the event dictionary according to the guide.

🐘 Queries

With time range

explain SELECT COUNT(DISTINCT "bulk_imports"."user_id") FROM "bulk_imports"
  WHERE "bulk_imports"."created_at" BETWEEN '2020-09-19 14:28:21.637489' AND '2020-10-17 14:28:21.637603'
  AND "bulk_imports"."user_id" BETWEEN 0 AND 999;

https://explain.depesz.com/s/l9tG

Aggregate  (cost=10.83..10.84 rows=1 width=8) (actual time=0.479..0.479 rows=1 loops=1)
   Buffers: shared hit=6 read=4
   I/O Timings: read=0.305
   ->  Index Scan using index_bulk_imports_on_user_id on public.bulk_imports  (cost=0.15..10.83 rows=1 width=4) (actual time=0.079..0.079 rows=0 loops=1)
         Index Cond: ((bulk_imports.user_id >= 0) AND (bulk_imports.user_id <= 999))
         Filter: ((bulk_imports.created_at >= '2020-09-19 14:28:21.637489+00'::timestamp with time zone) AND (bulk_imports.created_at <= '2020-10-17 14:28:21.637603+00'::timestamp with time zone))
         Rows Removed by Filter: 0
         Buffers: shared hit=1

Without time range

explain SELECT COUNT(DISTINCT "bulk_imports"."user_id") FROM "bulk_imports"
  AND "bulk_imports"."user_id" BETWEEN 0 AND 999;

https://explain.depesz.com/s/tZZ8

Aggregate  (cost=10.81..10.82 rows=1 width=8) (actual time=0.038..0.039 rows=1 loops=1)
   Buffers: shared hit=1
   ->  Index Only Scan using index_bulk_imports_on_user_id on public.bulk_imports  (cost=0.15..10.79 rows=7 width=4) (actual time=0.021..0.021 rows=0 loops=1)
         Index Cond: ((bulk_imports.user_id >= 0) AND (bulk_imports.user_id <= 999))
         Heap Fetches: 0
         Buffers: shared hit=1

🔍 Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• ~~Label as security and @ mention @gitlab-com/gl-security/appsec~~
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team