First bits of the Context overhaul.

Reintroduces separate contexts for users, channels, connections (now split into sessions and connections) and user-channel associations.
It builds which is as much assurance as I can give about the stability of this commit, but its also the bare minimum of what i like to commit sooooo
A lot of things still need to be broadcast through events throughout the application in order to keep states consistent but we'll cross that bridge when we get to it.
I really need to stop using that phrase thingy, I'm overusing it.

SharpChat.SQLite/SQLiteUserPermissionsConverter.cs

@@ -1,3 +1,5 @@
+using SharpChat.Users;
+
 namespace SharpChat.SQLite;
 
 public static class SQLiteUserPermissionsConverter {