OCS route for thumbnails

This replaces https://github.com/owncloud/core/pull/10805, changes:

- Code style fixed
- Exception thrown in case file is not found
- Route does not use GET parameters anymore

@georgehrke 
Not sure. This is OCS and not really REST. - I'd prefer to keep it this way.

@karlitschek What do you think? - You've written that protocol, not me ;-)