feat(calendar): single Default personal with guards and backfill

Ensure title Default; reject second personal and delete/type-change of
the sole personal; migrate extras to commercial or soft-delete.

Refs EventHub/EventHubBack#64

src/handlers/handler_calendars.erl

@@ -140,6 +140,8 @@ create_calendar(Req) ->
                   handler_utils:send_json(Req2, 201, Response);
                 {error, subscription_required} ->
                   handler_utils:send_error(Req2, 402, <<"Subscription required for commercial calendar">>);
+                {error, personal_exists} ->
+                  handler_utils:send_error(Req2, 409, <<"personal_exists">>);
                 {error, user_inactive} ->
                   handler_utils:send_error(Req2, 403, <<"User account is not active">>);
                 {error, {content_banned, _Words}} ->