Redesign the private sharing view and user flow to be similar to recent changes in public links

[davigonz]

I've been thinking about how to change the current private sharing to be more consistent with the rest of the sharing view and also include the new expiration date field which is requested in #1973

The current way to share a file/folder privately is by creating it without setting permissions and after that, updating the permission options. That was the previous behaviour in public links as well but we've changed this when we released multiple link shares to allow the user to create a public link with all the possible options and let him update it later.

Therefore, I propose you to use the same approach with Private Shares (Users/Groups). I include some mockups below.

1. Private share creation

Press + button	Press search bar

Select user/group and save	Private share properly saved

2. Private share edition

Press angle button	Edit private share

[davivel]

To discuss for iOS too.

[michaelstingl]

@davigonz @davivel @jesmrec Expiration date for user/group shares planned for oC server 10.0.5: https://github.com/owncloud/enterprise/issues/2001#issuecomment-332617932