Eager loading in notifications endpoint

[SychO9]

Split off from flarum/framework#2637

This is when hitting the endpoint /api/notifications?page[offset]=0&page[limit]=50&include=subject (extreme to be able to properly see uneager loaded relations).

There is improvement to be made, the difference between this endpoint and the others, is that the N+1 queries are mainly coming from the subject polymorphic relationship, for example subject.tags or subject.discussion.tags. Best way to go about it is to provide a loadMorph method to the ApiController extender to allow extensions (such as tags) to eager load the relations necessary.

[stale]

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