Removed welcome message + some more tests

Removed the welcome message from tasks and added some new tests
Issue https://github.com/myvoice-nigeria/myvoice/issues/207
This could easily get confusing in a few weeks/months. I think you should probably replace the column (e.g., a boolean ``survey_scheduled`` value). If the welcome message may come back in the near future, it might make more sense to leave it as-is but put a large/obvious comment in the ``models.py`` file explaining what ``welcome_sent`` actually means.