API: Include Relationships in Notification

[DagAgren]

It would be very nice if a Notification object contained the Relationships for the account in question. This would allow sorting notifications into regular and high-priority ones from people you follow or mutuals. Personally, I want to add a view to my client that shows notifications per mutual, for instance.

It can be done now by sending off a request to check for relationships for each notification, or by keeping a cached list of accounts, but that's inefficient and not very robust.

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• [x] I searched or browsed the repo’s other issues to ensure this is not a duplicate.
• [ ] This bug happens on a tagged release and not on master (If you're a user, don't worry about this).

[beatrix-bitrot]

this sounds cool and good

@Gargron what do you think?