Display instance follow/following relationships in instance view (partial implementation of 648)

What does this PR do? (Please give us a brief description of what this PR does.)

Resolves part of 648. Did not see that Hazel already claimed it, so feel free to disregard, or just use this MR as inspiration for the final design.

Adds two tabs to the instance view:

• Following - shows local users -> remote user follows
• Followers - shows remote users -> local user follows

Changes federation/following and federation/followers to also pack the follower user, otherwise we'd have to fetch every user separately.

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• 1c3495ef - Make including followers in response a parameter

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changed title from Display instance follow/following relationships in instance view to Display instance follow/following relationships in instance view (resolves #648)

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mentioned in merge request !650 (closed)

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• 46e53e29 - Parameterize followee as well

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I just noticed that without a way to search for users this line in the issue

"This is interesting to view, as a curiosity. Let's say that I want to see who follows who. What the most popular instance users are on a remote instance from our perspective"

Would become very hard to do as you would never know how many followers/following relations there are and how long you could end up scrolling just to finally find the user you were looking for.

That's a good idea for a future MR, but out of scope here imo. Would require an entirely new endpoint + functionality.

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changed title from Display instance follow/following relationships in instance view (resolves #648) to Display instance follow/following relationships in instance view (partial implementation of 648)