Check and fix N+1 queries for board issues

When querying issues for boards, we have a couple of N+1 issues when

1. requesting author field and 2. board's parent is group for projects.

@acroitor initially discovered the issue and noted in !71164 (comment 690105115):

I think I've also noticed an N+1 on author and project you may want to check it and see if we can preload the 2 associations.

It's highly likely that we have N+1 for more fields like assignees.