Do only follow HTTP and HTTPS redirects

We do not want to follow redirects to other protocols since they might allow an adversary to bypass network restrictions. (i.e. a redirect to ftp:// might be used to access files of a FTP server which might be in a secure zone and not be reachable from the net but from the ownCloud server)

Get final redirect manually using get_headers()

Migrate to HTTPHelper class and add unit tests

Conflicts:
	apps/files/ajax/newfile.php
	lib/private/files/storage/dav.php
	lib/private/server.php
	lib/private/util.php
	lib/public/iservercontainer.php
Merge pull request #11248 from owncloud/11032-backport-stable6

Do only follow HTTP and HTTPS redirects