Add serious business string for no calendar entries
Summary:
Allow boring humans to understand that a user may have events but they might not be visible to the current viewer when looking at a user's Calendar widget on a user profile by providing the string "No visible events." when in serious business mode instead of "Clear sailing ahead."
Also sneak in some PhpDoc while I was poking this anyway.
Test Plan:
* Enable http://phorge.localhost/config/edit/phabricator.serious-business/
* Install Calendar application at http://phorge.localhost/applications/view/PhabricatorCalendarApplication/
* Go to http://phorge.localhost/p/user/ and look at the "Calendar" box on the right.
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Differential Revision: https://we.phorge.it/D26225