OStatusPlugin: fix ensureProfile catch-22 in onCheckActivityAuthorship()

OStatusPlugin: fix ensureProfile catch-22 in onCheckActivityAuthorship()

Use profile URL (not URI), like elsewhere.

Profile::getUri() doesn't actually do anything useful, here--
it does nothing unless a plugin (like OStatus) is already
able to resolve the Profile into a backend object (e.g.: an Ostatus_profile).

If we might not already have an Ostatus_profile for a given Profile,
then we need to use $profile->getUrl() and fetch the data from that URL.