Query related epics of an epic

Related to #202431 (closed).

Similarly to Issue#related_issues, we need to be able to query all epics related to a given epic.

Requirements

1. Check for the user's permissions to read them We could implement a new method Ability.epics_readable_by_user that iterates every epic with readable_by?(user). Similarly to Ability.issues_readable_by_user.

As with most permission checks, the performance impact is a concern and should be evaluated.

Consider comments in the original discussion by @jprovaznik:

Click to expand

I think the most probable impact is there will be an N+1 issue when checking permission for each of the target epic, but given that this should be a small number (similar as for related issues), I think it's acceptable.

If I try something like this:

DeclarativePolicy.user_scope do
  Epic.all.preload(:group).select { |epic| Ability.allowed?(user, "read_epic", epic) }
end

Then the only N+1 query I hit locally is:

  SamlProvider Load (0.2ms)  SELECT "saml_providers".* FROM "saml_providers" WHERE "saml_providers"."group_id" = 28 > LIMIT 1 /*application:console,db_config_name:main,line:/ee/app/models/ee/group.rb:202:in `block in root_saml_provider'*/

But there is not much we can do about this one.

2. Update the number of blocking epics of an epic when this number changes (see EE::Issue#update_blocking_issues_count!)

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