Optimize checking if a user can read multiple issues
## What does this MR do?
This optimizes various parts of the code so it can more efficiently check if a user can read a list of issues.
## Are there points in the code the reviewer needs to double check?
Yes, in particular `Ability.issues_readable_by_user` should be checked to make sure it correctly allows/restricts access to issues.
## Why was this MR needed?
Currently the general approach to checking if one can read an issue is to iterate over the issues to check and call `can?(user, :read_issue, issue)` for every issue. This is not efficient as the same work has to be done for every issue.
## What are the relevant issue numbers?
* #15607
* #17463
See merge request !5370
