Add admin page for batched background migrations

What does this MR do?

Adds a page in the admin section to show the current progress of batched background migrations

Queries

Query for successful rows per migration

SELECT "batched_background_migration_jobs"."batched_background_migration_id", SUM(batch_size) AS successful_rows
FROM "batched_background_migration_jobs"
WHERE "batched_background_migration_jobs"."status" = 3 AND 
  "batched_background_migration_jobs"."batched_background_migration_id" IN (1, 2, 3)
GROUP BY "batched_background_migration_jobs"."batched_background_migration_id"

Time: 48.262 ms  
  - planning: 0.994 ms  
  - execution: 47.268 ms  
    - I/O read: 41.324 ms  
    - I/O write: N/A  
  
Shared buffers:  
  - hits: 30 (~240.00 KiB) from the buffer pool  
  - reads: 200 (~1.60 MiB) from the OS file cache, including disk I/O  
  - dirtied: 0  
  - writes: 0  

 Aggregate  (cost=0.29..1050.01 rows=7 width=16) (actual time=43.069..47.182 rows=3 loops=1)
   Group Key: batched_background_migration_jobs.batched_background_migration_id
   Buffers: shared hit=30 read=200
   I/O Timings: read=41.324
   ->  Index Scan using index_batched_jobs_on_batched_migration_id_and_status on public.batched_background_migration_jobs  (cost=0.29..1003.58 rows=9271 width=12) (actual time=7.929..45.370 rows=9248 loops=1)
         Index Cond: ((batched_background_migration_jobs.batched_background_migration_id = ANY ('{1,2,3}'::bigint[])) AND (batched_background_migration_jobs.status = 3))
         Buffers: shared hit=30 read=200
         I/O Timings: read=41.324

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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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

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• 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

Related to #326760 (closed)