[BUGFIX] Fixed date format bug

See:
https://sourceforge.net/p/tellmatic/bugs/96/
[BUGFIX] Read newsletter content in subscription process

The mails in the subscription process are now sent with content. This
patch adds a missing parameter to the $NEWSLETTER->getNL() so that
the body fields will be filled in the returned array.
[BUGFIX] Do no add empty parts to emails

If a part of an email (text or html) is empty the part header will not
be added to the email any more. This will prevent mail clients from
displaying empty mails for text only emails.
[BUGFIX] Display correct message on double opt in

When a user registers a new address and double opt in is active then
the correct double opt in message will now be displayed instaed of the
invalid greeting message.
[FEATURE] Allow redirection in subscribe process

It is now possible to configure redirections for the different use cases
in the subscribe form (subscription, double opt in, update).

This change uses the available message fields and checks if the message
starts with "REDIRECT:"
[TASK] Moved constant template initialization

The initialization of temlate values that are based on global constants
was moved to the new method in Class_Tpl::setConstantParseValues().
[BUGFIX] Fill constant template values on subscribe success

When the success form is displayed during the subscribe process the
template values that are based on global constants are now filled
with their corresponding values.
[FEATURE] Double opt in error message

When the user tries to confirm his subscription but the submitted
data is invalid a configurable error message is now displayed using an
additional form temlate (/files/forms/Form_[n]_failure.html).

Hint! The form failure template needs to be created by hand.
[TASK] Prevent multiple address confirmation

When the user confirms his address in the double opt in process the
activation code will be removed from the database. This makes it
impossible to use the code again.
[TASK] Re-send double opt in for waiting addresses

If a user fills out the subscribe form and his address is already
registered but waiting he will get the double opt-in message and email
again so that he is able to complete his subscription.
[BUGFIX] Display all debug messages on subscribe success

A lot of debug messages in the subscribe process are directly written
to the $OUTPUT variable which was overwritten with the subscribe success
form.

The form gets now appended so the debug messages are visisble again.