Add OCS capability to advertise whether a server allows public links

[PVince81]

curl http://root:admin@localhost/owncloud/ocs/v1.php/cloud/capabilities

Add a new attribute that tells whether public sharing is enabled.

@guruz @icewind1991 @schiesbn

[PVince81]

CC @rperezb @dragotin as this info can be interesting for the mobile and desktop clients

There's a PR from @rullzer here: #13692

[rperezb]

Thanks @PVince81, indeed it is.

[MTRichards]

If/when we get a fix, can we get a backport to 8.0 and 7.0.5 please? This is key for the mobile and desktop - as well as the info on whether a password is required for public links.

[rullzer]

@MTRichards I did not test it (yet) but I think my PR can be trivially backported to 7.0.5.
As for 8.0 it works against the current master.

[MTRichards]

Awesome sauce. Does it also advertise if a public link requires a password?

[rullzer]

Yes, and if we allow public uploads. The rest can be added as well (default expire date etc).
However there was some discussion where it had to be placed in the capabilitie

[davivel]

Sorry, Matt, we messed this a bit before. From the client POV would be great to have this backported, but is not really necessary for the problem we talked about.

The key point is in my comment in owncloud/client#2774 (comment) .

Thanks.

[dragotin]

I think the capabilities API should provide the followiing information, based on @rullzer s pull request:

1. Public link sharing enabled yes/no (already in the patch)
2. Password required (already in the patch)
3. Default expiration date yes/no, if set, the duration in days
4. Expiration date enforced yes/no
5. Mail sending for public link shares yes/no
6. Mail sending for user/group sharing yes/no
7. Restrict sharing to own group yes/no
8. Allow sharing to groups yes/no

No. 7. and 8. could also be handled by a server API that returns the list of users/groups that the client is allowed to share to.

We will currently not need the public upload information.