add nextcloud provider file

[Danielle Foré]

depends on https://bugs.launchpad.net/switchboard-plug-onlineaccounts/+bug/1588844

Sounds like at the moment nextcloud support card and caldav. Requires a url to point to the server, then of course the auth.

[Danielle Foré (danrabbit)]

From Lukas in slack:

So starting with the development release you can send a `POST` to `/token/generate` containing:

- user: Username
- password: User password
- name: Name of the client (for example "Most awesome Elementary Sync-thingy")

Then you will get back a 128 character random token in a JSON array. The token is mapped to an element called `token`. You can then use that token to authenticate against the APIs. You can do this by using it simply as a password and use the normal username.

 if the API by any case returns a cookie you should just resend this as well. We are for example considering adding Same Site Cookies as a security feature and in that case it would be awesome if the client would be clever enough to notice: "We got a cookie so we need to resend it".

The best way to test the implementation is probably to download a daily release from https://download.owncloud.org/community/daily/owncloud-daily-master.tar.bz2 and test against it.