Ostatus_profile::checkAuthorship(): throw ServerException when bogus non-authorship is detected

Similarly to what ActivityUtils::checkAuthorship does; try to ensure
that activities from ambiguous OStatus feeds (groups and peopletags)
that require explicit authorship don't get in without explicit authors.
Fix OStatus groups by making Ostatus_profile::localProfile() work for groups

We need to look up a feed profile for HandleFeedEntryWithProfile events,
regardless of whether they're an OStatus user, group, or something else;
this is the least hairy way of doing that--the alternative being
to keep spreading the same logic all over the calling code.

Theoretically, this change might allow OStatusGroups to be recorded
as the authors of activities if they pass through any authorless
activities; but that's why we have checkAuthorship().
Bad parameter count for checkAuthorship

At the same time we change this to call ActivityUtils::checkAuthorship
instead to let the retrieval/verification go through event handling.

rozzin (Joshua Judson Rosen) found this error. Thanks.