Download subm

Can now download submissions by appending `?download=true` to the submission url. Added in front end link for doing that too.
Yeah, I figured. 

There's no other way to duplicate the logic and add something after it?

Wait, why don't we just make a new method? Like a download method. So it'll be something like `/api/v1/submission/1/download`. That would make more sense, right...?