Fix share download

fixes download issue introduced by https://github.com/owncloud/core/pull/10755

This is a fairly large change please test thoroughly as im sure we missed some cases
I actually did take a look at this since I agree with that. The main problem here is that we want to use a route *without* prepending the appname in the URL. (`/s/` instead of `/apps/files_sharing/s`)

@Raydiation Do you have any hint how we can do this? Migrating should be easy once this is done.