🙈 *cough* *cough*

str_replace for $user substitution was converting the data type of
mountOptions to string. This fix prevents this to happen by making sure
only strings are processed by substitution.

Also added a int conversion when reading the watcher policy

This reverts commit 18b5966f.

Added option to disable autoscan of external storages
Mount option file system scan is now an int
Move priority field to avoid undefined field in storage options

All input elements inside the storage options block get parsed into
storage options.

Moving the priority field outside prevents it to appear in the storage
config, as expected. It is still parsed by special code.

* Only use public interfaces
  - Injected IPreview
* Added unit tests

Despite it's PHPDoc the function might return `null` which was not properly catched and thus in some situations the share was resolved to the sharing users root directory.

To test this perform the following steps:

* Share file in owncloud 7 (7.0.4.2)
* Delete the parent folder of the shared file
* The share stays is in the DB and the share via the sharelink is inaccessible. (which is good)
* Upgrade to owncloud 8 (8.0.2) (This step is crucial. The bug is not reproduceable without upgrading from 7 to 8. It seems like the old tokens are handled different than the newer ones)
* Optional Step: Logout, Reset Browser Session, etc.
* Access the share via the old share url: almost empty page, but there is a dowload button which adds a "/download" to the URL.
* Upon clicking, a download.zip is downloaded which contains EVERYTHING from the owncloud directory (of the user who shared the file)
* No exception is thrown and no error is logged.

This will add a check whether the share is a valid one and also adds unit tests to prevent further regressions in the future. Needs to be backported to ownCloud 8.

Adding a proper clean-up of the orphaned shares is out-of-scope and would probably require some kind of FK or so.

Fixes https://github.com/owncloud/core/issues/15097