Updates to template and tokens

The simplenews-newsletter-body.html.twig has changed. If you have customised this template, please update it based on the new default version. The variables have changed as follows:

1. New variable reason.
2. Variable unsubscribe_text can now be empty, and if so the token [simplenews-subscriber:unsubscribe-url] must not be used
3. Variable test_message can now be empty.
4. Variable key now takes an additional value extra which indicates an extra copy of the newsletter sent by an administrator. This variable should not be needed - instead check if test_message is empty.
5. Variable opt_out_hidden is deprecated - instead check if unsubscribe_text is empty.

The tokens have changed as follows:

1. The token [simplenews-subscriber:unsubscribe-url] will not be replaced if there isn't a valid subscriber (for example for test/extra messages). Check the variable unsubscribe_text is not empty before using it.
2. Remove [simplenews-subscriber:subscribe-url] token. It no longer has any purpose, all it can do is to put a subscribe link for a newsletter into that same newsletter.