This should be set to False, otherwise users with undecidable wiki usernames will be incorrectly linked to a missing page in `pages.py header_nav()`

Using [False, True, 'unknown'] as possible values for `wiki_account` is less robust than all strings ('unassociated', 'associated', 'unknown') because `wiki_account` is consumed as a boolean (and 'unknown' would evaluate to True).  Currently, reading `wiki_account` directly may lead to wrong results -- calls would always have to go through `associated_wiki_account`, which may not be apparent to other developers.  I would prefer the approach with explicit string compare.