* 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

Way quicker

* Also use the URLGenerator

In the past it seems the update permission was needed to be able to
rename shared mounts, but it doesn't seem to be the case any more.

Removing the "update" permission that used to be added automatically
fixes the read-only permission check when trying to overwrite a
read-only file over WebDAV.

Tests added to make sure that removing an expire date when this is
enforced is not allowed.

Previously the mount name was checked for uniqueness prior to inserting the
share. This caused problems, when two shares with the same name where done
or folder, mount point, local share with the same name was done, between
sending and accepting of the remote share

\OC\URLGenerator::imagePath is a really expensive operation due to all the I/O handling and can really benefit from caching.

Fix various PHPDoc issues in external storage app.

Added missing NoAdminRequired tag

Ref https://github.com/owncloud/core/pull/14929#discussion_r26501240