Prevent scroll up when click on links in sharing dropdown

When clicking on a link that points to "#" in the sharing dropdown, and
the current view is scrolled down, the browser will scroll the page up
to the top (anchor behavior).

This fix cancels the click event when clicking on "unshare" link or
"show crud" link.
Fixed label for sharing notification to be clickable

Fixed the sharing notification checkbox label in the file sharing
dropdown, which makes it possible to click on the label directly.
Prevent share dropdown to close too early with autocomplete

When clicking on an autocomplete result in the sharing dropdown,
it will mistakenly be detected as being clicked outside and would
trigger the sharing dropdown closing.

This fix adds a check to detect clicks inside the autocomplete and
prevent closing the dropdown in that case.

This fixes #4142
Merge pull request #5169 from owncloud/sharingdropdown-uxbugfixes

Sharing dropdown bug fixes