[FEATURE] Unsubscribe settings for forms

In the form settings an unsubscribe message and an unsubscribe newsletter
can now be selected. This will be used in a later commit that introduces
a personalized unsubscribe process.
[FEATURE] Generate plain subscribe / unsubscribe URLs

Currently the unsubscribe und subscribe URL in the templates are only
working for the HTML versions of the mails because the HTML entities
are encoded (& => &amp;).

Since this is not working in plain text mails two new template markers
were introduced for use in the plain text mails:
UNSUBSCRIBE_URL_PLAIN and SUBSCRIBE_URL_PLAIN