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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