Multiple fixes to sharing overview

- scrolling to next page now correctly renders share icons
- reshareing/unsharing a reshare will now still keep "Shared by" label
Use recipient display names when updating shares in the UI

Since OC.Share didn't have any array containing the list of shares for
the current file, OC.Share.currentShares has been introduced to contain
the full share item structure instead of the reduced one
OC.Share.itemShares.

The event "sharesChanged" is now passing OC.Share.currentShares, which
itself includes the display name to be displayed for the recipients in
the action icon.
Do not update action icon recipients for file list

Since the files app doesn't have the recipient information on load it
cannot initially render the recipients in the action icons.

For this reason the action icon will not be updated with the recipients
after changing the shares, for consistency.