Fix double encoding of html diff output in syndicated feeds and subscrition emails

de/de-informal language updates for several components.

Rename ad_usernamepassword to admin_username/password

again)

canDo getUsers to false when unavailable

This reverts commit ba09710a.

Conflicts:
	lib/plugins/authad/lang/de/settings.php

Correctly identify AD account

server side resizing now needs a security hash which we can' provide in
the installer, yet.

FS#2748, fix double encoding of html entities in subscription email diffs (updates subscription.php for changes to Diff class introduced in PR#179)

FS#2738, fix double encoding of html entities in RSS feeds (updates feed.php for changes to Diff class introduced in PR#179)

Before this fix the debug parameter always prevent the GIF from being
sent even though no actual debug information was returned unless the
allowdebug configuration option was set. Now the GIF is only disabled if
debugging is actually allowed.

prevent third-party requests from using fetch

process a crop as a resize when possible

backward compatibility for old authtype settings

install.php: Add pack() to required functions (FS#2761)

mechanism supporting "authtype deprecated" message for superusers only

de/de-informal language updates for several components.

for completeness (& defensive coding), test ['show'] against MSG_PUBLIC in case its redefined to a non-zero value

add capability to restrict recipients of dokuwiki 'msg' alerts.  This is useful where message is added to the queue before authentication is initialized