Fix blockedByCount GraphQL field

What does this MR do and why?

In !128152 (merged) we added the GQL aggregator WorkItems::LazyLinksAggregate for blockingCount and blockedByCount fields but we didn't resolve the method links_aggregate which results in an internal server error when calling the count fields:

undefined method `to_i' for #<Gitlab::Graphql::Aggregations::WorkItems::LazyLinksAggregate:0x00007f572a9dce10 @issuable_id=132216329, @link_type="blocked", @block=#<Proc:0x00007f572a9dcd98 /srv/gitlab/ee/app/graphql/ee/types/work_items/widgets/linked_items_type.rb:33>, @lazy_state={:pending_ids=>{"blocked"=>#<Set: {132216329}>, "blocking"=>#<Set: {}>}, :loaded_objects=>{"blocked"=>{}, "blocking"=>{}}}> Did you mean? to_s

This MR adds lazy_resolve for this aggregator so loaded_objects is populated with the counts and the block is called correctly for the coun fields.

Additionality, it's included in this MR a small fix for Project#linked_work_items_feature_flag_enabled? because the feature flag was not working when scoped to a project.

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Related to #416768 (closed)