Add index to improve counting of open issues

What does this MR do and why?

Adds an index to improve the performance of counting open issues of a project. This query is triggered every time an issue is created or changes state.

This MR only prepares the index for weekend creation on GitLab.com: https://docs.gitlab.com/ee/development/adding_database_indexes.html#create-indexes-asynchronously

Query:

SELECT COUNT(*) AS count_all, "issues"."confidential" AS issues_confidential
FROM "issues"
WHERE ("issues"."state_id" IN (1)) AND "issues"."project_id" = 19923279
GROUP BY "issues"."confidential"

Note: The query right now in master includes an issue_type filter, but that will be removed by !89118 (merged)

Before:

 HashAggregate  (cost=51322.17..51322.19 rows=2 width=9) (actual time=249.499..249.501 rows=2 loops=1)
   Group Key: issues.confidential
   Buffers: shared hit=83043
   I/O Timings: read=0.000 write=0.000
   ->  Index Scan using index_issue_on_project_id_state_id_and_blocking_issues_count on public.issues  (cost=0.56..51142.76 rows=35883 width=1) (actual time=0.047..221.147 rows=91423 loops=1)
         Index Cond: ((issues.project_id = 19923279) AND (issues.state_id = 1))
         Buffers: shared hit=83043
         I/O Timings: read=0.000 write=0.000

Time: 253.770 ms  
  - planning: 4.148 ms  
  - execution: 249.622 ms  
    - I/O read: 0.000 ms  
    - I/O write: 0.000 ms  
  
Shared buffers:  
  - hits: 83043 (~648.80 MiB) from the buffer pool  
  - reads: 0 from the OS file cache, including disk I/O  
  - dirtied: 0  
  - writes: 0  

Sometimes the query is fast like above, but sometimes it times out as seen in the linked issue.

After:

 Aggregate  (cost=0.56..3454.36 rows=2 width=9) (actual time=0.243..55.344 rows=2 loops=1)
   Group Key: issues.confidential
   Buffers: shared hit=4931
   I/O Timings: read=0.000 write=0.000
   ->  Index Only Scan using idx_open_issues_on_project_id_and_confidential on public.issues  (cost=0.56..3274.93 rows=35882 width=1) (actual time=0.150..40.636 rows=91423 loops=1)
         Index Cond: (issues.project_id = 19923279)
         Heap Fetches: 4986
         Buffers: shared hit=4931
         I/O Timings: read=0.000 write=0.000

Time: 55.810 ms  
  - planning: 0.421 ms  
  - execution: 55.389 ms  
    - I/O read: 0.000 ms  
    - I/O write: 0.000 ms  
  
Shared buffers:  
  - hits: 4931 (~38.50 MiB) from the buffer pool  
  - reads: 0 from the OS file cache, including disk I/O  
  - dirtied: 0  
  - writes: 0 

Migration output

main: == 20220602111923 PrepareIndexForOpenIssuesCount: migrating ===================
main: -- index_exists?(:issues, [:project_id, :confidential], {:where=>"state_id = 1", :name=>"idx_open_issues_on_project_id_and_confidential", :algorithm=>:concurrently})
main:    -> 0.0069s
main: -- add_index_options(:issues, [:project_id, :confidential], {:where=>"state_id = 1", :name=>"idx_open_issues_on_project_id_and_confidential", :algorithm=>:concurrently})
main:    -> 0.0001s
main: == 20220602111923 PrepareIndexForOpenIssuesCount: migrated (0.0177s) ==========

ci: == 20220602111923 PrepareIndexForOpenIssuesCount: migrating ===================
ci: -- index_exists?(:issues, [:project_id, :confidential], {:where=>"state_id = 1", :name=>"idx_open_issues_on_project_id_and_confidential", :algorithm=>:concurrently})
ci:    -> 0.0079s
ci: -- add_index_options(:issues, [:project_id, :confidential], {:where=>"state_id = 1", :name=>"idx_open_issues_on_project_id_and_confidential", :algorithm=>:concurrently})
ci:    -> 0.0001s
ci: == 20220602111923 PrepareIndexForOpenIssuesCount: migrated (0.0118s) ==========

main: == 20220602111923 PrepareIndexForOpenIssuesCount: reverting ===================
main: == 20220602111923 PrepareIndexForOpenIssuesCount: reverted (0.0085s) ==========

ci: == 20220602111923 PrepareIndexForOpenIssuesCount: reverting ===================
ci: == 20220602111923 PrepareIndexForOpenIssuesCount: reverted (0.0085s) ==========

Related to #363603 (closed)

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