Fix Twitter Card defaults and fallback

[fritzmg]

This PR fixes the erroneous defaults for hofff_st_twitter_type in the DCA - and the fallback for hofff_st_twitter_type in the TwitterCardsFactory.

[fritzmg]

Note: the following SQL queries might be necessary afterwards, in order to fix the wrong default values in the database:

UPDATE tl_news SET hofff_st_twitter_type = '' WHERE hofff_st_twitter_type = 'website';
UPDATE tl_page SET hofff_st_twitter_type = '' WHERE hofff_st_twitter_type = 'website';
UPDATE tl_faq SET hofff_st_twitter_type = '' WHERE hofff_st_twitter_type = 'website';
UPDATE tl_calendar_events SET hofff_st_twitter_type = '' WHERE hofff_st_twitter_type = 'website';

[dmolineus]

Thanks @fritzmg