Collections can only be found from a file's page, no list to be found

[maxine]

The collections that a user may have are nowhere to be found but on the page of a specific file. It would be very handy to list a user's collections on the userpage or somesuch.

example.org/u/collection/acollection shows the page for the collection (and is only linked from a file's page), but example.org/u/user/collection or /collections returns 404

e.g.:
​http://latticeqcd.net:6543/u/maxine/collection/wallpapers/
​http://latticeqcd.net:6543/u/maxine/collection/
​http://latticeqcd.net:6543/u/maxine/collections/

[Stefano Zacchiroli]

This is a duplicate of #526 (which now contains a patch for this issue).