Overhaul in order to simplify workflow, and prepare for multilingual sub...

...classing.

- Fixed jquery initialization in enl_edithelper.js and make it work again.
  It didn't hide the user selection fields.

- Status and error messsages show up for anon users (was broke).

- Implemented that subscribers and recipients have a language. This is also
  queable via index and the affecting page templates have been adjusted.

- Made a checkbox in the send issue form to exclude all external subscribers.

- Split up the send method for better hookability.

- Reworked the whole issue workflow. See README.

- Added info on several newsletter fields, that changing settings does not
  affect already existing issues for that newsletter.

- Fixed enl_edithelper.py work again.

- Added 2 new constants to the config.py for hookability and adjusted the
  affected files to look up those constants.

- Fixed A subscriber can now unsubscribe himself, if he is not logged in.

- Reworked the salutation mapping (prepared for multilingual newsletter)

@jensens @agitator please have a look