Fix bonus calculation crash and time entry form responses

[travismolick]

Fixes two bugs found while writing the tests in #78:

• TimeEntry::calculateBonusTime() crashed with "Attempt to read property "department_id" on null" when called without preloaded bonuses. In that case it queries the bonuses itself, already filtered by department, so they have no pivot. It now checks the pivot only when there is one. Nothing in the app calls it that way today, so this prevents a future crash rather than fixing a live one.
• VolunteerController::storeTimeEntry() and destroyTimeEntry() were declared to return JsonResponse, but return a redirect for non-JSON requests, so form submits threw a TypeError (500). The return types now allow RedirectResponse.

Each fix has a test (TimeEntryBonusTest, TimeEntryFormRedirectTest) that fails without it.