Populate issues blocking_issues_count field

On !37368 (merged) we added blocking_issues_count field to issues table.
Now we have to backfill issues that are blocking others and still have 0 value on the column.

This can be done with a background migration. I added the record count that needs to be updated below:

For BLOCKS link type

SELECT COUNT(*) FROM issue_links
  LEFT OUTER JOIN issues ON issue_links.source_id = issues.id
  WHERE issues.blocking_issues_count = 0 AND issue_links.link_type = 1

 count 
-------
 12985
(1 row)

for IS_BLOCKED_BY link type

SELECT COUNT(*) FROM issue_links
  LEFT OUTER JOIN issues ON issue_links.target_id = issues.id
  WHERE issues.blocking_issues_count = 0 AND issue_links.link_type = 2
  
 count 
-------
 33317
(1 row)