Sharing ux improvements

As discussed here https://github.com/owncloud/core/issues/8344

- The admin settings now only show "Set default expiration date", if it is checked we will show the input field and the "enforce expire date" checkbox.

- If you hover over the "set expire date" checkbox or the date-picker you will get following messages: If default expire date is set "By default shares will expire after 7 days" and if it is also enforced "Shares will expire not later than 7 days after they were shared".

cc @jancborchardt please have a look. Thanks!