Enhancement: Shares overview

We've merged the three shares navigation items into one central "Shares" item, 
with a toggle to switch between the three different kinds of shares (incoming, outgoing, links).
In the process, we have also renamed the "All files" page to the "Personal" page, indicating that 
this is the user's personal space since shares (and potentially other shared spaces) live elsewhere.

https://github.com/owncloud/web/issues/6440
https://github.com/owncloud/web/issues/6570
https://github.com/owncloud/web/pull/6512
https://github.com/owncloud/web/pull/6573
