Implement upload and get file endpoints for APIv4 (#5396)

* Implement POST /files endpoint for APIv4

* Implement GET /files/{file_id} endpoint for APIv4

api4/api.go

@@ -143,6 +143,7 @@ func InitApi(full bool) {
 	InitTeam()
 	InitChannel()
 	InitPost()
+	InitFile()
 	InitSystem()
 
 	app.Srv.Router.Handle("/api/v4/{anything:.*}", http.HandlerFunc(Handle404))