Hello Phoenix,
Apologies for the delayed response. The online subsystems in UE4 are a set of cross-platform interfaces to interact with the online services provided by various platforms - for example, if you are a Steam developer with access to the SDK, you can use the online subsystem to login to Steam, perform matchmaking, award achievements, and so on.
The online subsystems do expose per-user cloud storage, which is supported on Steam. Take a look at IOnlineUserCloud in OnlineUserCloudInterface.h, but keep in mind this is generally used to store data such as save game files in the cloud.
Setting up a custom server outside of the provided online subsystems to handle user accounts is unfortunately a bit beyond the scope of UE4 right now. You will probably need to set up your own server for this and add code to your game client to communicate with this server. Since you’re already using Photon it may be possible to write a UE4 plugin that acts as a Photon client, communicating directly with the Photon server - the Photon developers may be able to help out here.