Clean up User class

[peppy]

• Depends on Remove all remaining calls to ToBeatmapInfo/ToBeatmapSetInfo #15463
• Depends on Rename User to APIUser and move to correct namespace #15505

For now I haven't created a non-API version of this class (apart from the simple realm one). We can iterate on that if/when required.

conflicts (nice timing)

[bdach]

So I started reviewing this, but I have a big problem with b9983ad. That commit generates 99% of this diff and contains multiple complications that were not explained anywhere, namely:

• That APIUser already existed but was unused, and was co-opted for the rename instead.
• That not only User was renamed, but that other related API classes were also renamed (APIPlayStyle, APIUserScoreAggregate)
• That commit also includes the introduction of IUser.

All of the above make reviewing this as-is a pretty dreadful experience because I'm basically discovering half of the satellite changes by browsing through a 200-file diff. If it's not too much trouble I'd much prefer that it was split to a separate pull, with split commits for each individual refactoring operation performed.

Normally I wouldn't ask for this, but when the diffstat is 2k lines long...

[peppy]

can split out those commits if you need. no real explanation though, just matching other recent changes

[bdach]

I guess I could try and power through with the changes as they are but splitting that part better would make it a lot easier for me to review this.

[smoogipoo]

I managed to get a crash with this somewhere in the beatmap overlay, but the error was non-descript:

Unhandled exception. System.NullReferenceException: Object reference not set to an instance of an object.
   at osu.Framework.Graphics.Containers.GridContainer.layoutContent()
   at osu.Framework.Graphics.Containers.GridContainer.Update()
   at osu.Framework.Graphics.Drawable.UpdateSubTree()
   at osu.Framework.Graphics.Containers.CompositeDrawable.UpdateSubTree()
   at osu.Framework.Graphics.Containers.CompositeDrawable.UpdateSubTree()

And I also couldn't repro it. So lgtm.

[peppy]

Worrying, but hopefully not related to this PR alone...