docs: fix dead links in the box, sftp and webdav docs

The Box OAuth page, the SSH CBC paper on isg.rhul.ac.uk and the
nextcloud/nextcloud-snap issue all 404 now. Point them at the current
Box refresh token guide, the IEEE S&P 2009 copy of the paper and the
moved nextcloud-snap/nextcloud-snap repo.

backend/webdav/webdav.go

@@ -1783,7 +1783,7 @@ func (o *Object) updateSimple(ctx context.Context, body io.Reader, getBody func(
 		GetBody:       getBody,
 		Body:          body,
 		NoResponse:    true,
-		ContentLength: &size, // FIXME this isn't necessary with owncloud - See https://github.com/nextcloud/nextcloud-snap/issues/365
+		ContentLength: &size, // FIXME this isn't necessary with owncloud - See https://github.com/nextcloud-snap/nextcloud-snap/issues/365
 		ContentType:   contentType,
 		Options:       options,
 		ExtraHeaders:  extraHeaders,

docs/content/box.md

@@ -126,9 +126,10 @@ have just set.
 
 ### Invalid refresh token
 
-According to the [box docs](https://developer.box.com/v2.0/docs/oauth-20#section-6-using-the-access-and-refresh-tokens):
+According to the [box docs](https://developer.box.com/guides/authentication/tokens/refresh/):
 
-> Each refresh_token is valid for one use in 60 days.
+> A Refresh Token is valid for 60 days and can be used to obtain a new Access
+> Token and Refresh Token only once.
 
 This means that if you
 

docs/content/sftp.md

@@ -1454,7 +1454,7 @@ default, due to security concerns. This can be re-enabled on a
 per-connection basis by setting the `use_insecure_cipher` setting in
 the configuration file to `true`. Further details on the insecurity of
 this cipher can be found
-[in this paper](http://www.isg.rhul.ac.uk/~kp/SandPfinal.pdf).
+[in this paper](https://conferences.computer.org/sp/pdfs/sp/2009/oakland2009-01.pdf).
 
 SFTP isn't supported under plan9 until [this
 issue](https://github.com/pkg/sftp/issues/156) is fixed.

docs/content/webdav.md

@@ -384,7 +384,7 @@ ownCloud supports modified times using the `X-OC-Mtime` header.
 
 This is configured in an identical way to ownCloud.  Note that
 Nextcloud initially did not support streaming of files (`rcat`) whereas
-ownCloud did, but [this](https://github.com/nextcloud/nextcloud-snap/issues/365)
+ownCloud did, but [this](https://github.com/nextcloud-snap/nextcloud-snap/issues/365)
 seems to be fixed as of 2020-11-27 (tested with rclone v1.53.1 and Nextcloud
 Server v19).