Add ContextUser to http request context (#18798)

This PR adds a middleware which sets a ContextUser (like GetUserByParams before) in a single place which can be used by other methods. For routes which represent a repo or org the respective middlewares set the field too.

Also fix a bug in modules/context/org.go during refactoring.

modules/context/repo.go

@@ -439,6 +439,7 @@ func RepoAssignment(ctx *Context) (cancel context.CancelFunc) {
 		}
 	}
 	ctx.Repo.Owner = owner
+	ctx.ContextUser = owner
 	ctx.Data["Username"] = ctx.Repo.Owner.Name
 
 	// redirect link to wiki