drop any fallback code related to curl - refs https://github.com/owncloud/core/pull/14838#issuecomment-78586447

This is required because the PDF Viewer itself is embedded using an iframe from the same domain. The default policy is blocking this.

Going on further, we have to come up with a solution in the future how to handle previews by applications, one example might be that they call their own endpoint and not the generic share page to allow applications to have full control over how to display previews.

Anyways, to test this behaviour use a decent newer browser (such as Chrome 41) and share a PDF file, obviously the PDF viewer needs to be enabled as well. Without this patch publicly shared PDF files should not get previewed and an error is thrown. (if it isn't then your browser is probably not obeying our Content-Security-Policy and you might consider switching to another one ;))

Since aa821ecc leading and trailing slashes are
trimmed in the file cache paths

We were using the SMB and SFTP backends, which since Icewind's new SMB library
results in an actual attempted connection just by constructing the storage.
This commit replaces all references to real storages (except when necessary)
with a dummy storage class.

Fixes https://github.com/owncloud/core/issues/13326

Whenever versions need to be rescanned, only do it once per PHP request.
Happens whenever multiple files need to be expired.

* Throw 400 when a share is created or updated without read permissions
* Added unit tests