Allow you to specify None for the 'head' section of table_like.

If you say None, then all header tests are skipped.
Add dl_like to rubble.

Checks a definition list (<dl>) for contents. Pass it a list of tuples, like this:

dl_like([
    ('title', 'definition'),
    (None, 'definition'),
    None,
    ('title', 'definition')
])

Anywhere that None is used, those checks will be skipped.
Fix typo in the audit log message for when people sign out.