For this move the detection and validtion of a ReturnTo parameter
from the cleardiscochoies method into a helper method so it can
be reused. Also update the dos for this change.

Co-authored-by: Thijs Kinkhorst <thijs@kinkhorst.com>

strftime is deprecated and will be removed in PHP 9. SSP 2.0 is our
chance to change this format as it's exposed to installers of SSP.

The approch is to keep it simple and just change the format to PHP's
built in date() function which remains supported. Also we do not try
to localize the logging date/timestamps anymore. This matches the syslog
approach that this format tried to mimic. We are using the day without
leading zeroes now also to be a bit closer to how syslog does it.

We remove the functionality to see all auth sources and to test them
publically. Testing the auth sources can be done inside the admin
module.

Added is a configuration option that allows the administrator to specfy
another place to redirect to for people who land on the front page.

Fix spellings and typos

Added ability to skip validating the AuthnRequest response destination against metadata if the message is signed and the endpoint doesn't already exist in the SP metadata.

Hook 'metadata_hosted' and 'frontpage' are no longer called so do
not register hooks for them. Update the docs. Remove useless call
to configpage hook on admin module Test page (likely a copy-paste
error).

Now also considers any attributes.po in the configured theme, if any.
Makes the attributes.extradictionary config setting obsolete.

We could use it everywhere, but it's better to reduce the already big
amount of configuration options the admin is confronted with than the
small problem this option solves.