Distinguish non-all-day alerts

[digitalcora]

Currently we round off all alert time periods to whole days, which results in a more readable and less obtrusive presentation on calendars, but requires users to dig into the (sometimes verbose) description to notice when a disruption is actually not happening all day. We could do better with the presentation of these "non-all-day" disruptions.

• On the web calendar, their color blocks could have a striped or otherwise visually "partial" appearance.
• In the iCal feed, we could detect whether the time periods all have the same start/end time-of-day and try to give the title a corresponding prefix like "Evenings" or "From 8:45pm", with a generic fallback. (Really, almost all rapid transit disruptions that aren't all-day are specifically 8:45pm to end-of-service; maybe it's only worth recognizing this one pattern.)