feat(AlertController): put new features on the subway alerts page

[thecristen]

Summary of changes

Asana Ticket: Service Alerts page | Refine Station and Service Alerts section and Service Alerts page | Add "Current Status" section for subway

Most of the changes here were adding an additional sub-layout for the content of the alert page. Everything's the same between modes except for:

• the icon shown next to each route (for subway it's the subway_route_pill, all others route_icon)
• the <h2> shown above the list of alerts, as there's no heading shown for non-subway
• I changed the verbiage of the "no alerts here" message for subway to read more like "no other alerts here", since there might be other alerts covered by the Current Status or Planned Disruptions sections
• and of course, the new sections are only included for subway

Outside of the main content, the timeframe filters are also omitted from the sidebar for subway only.

Hopefully the structure is clear enough. Please omit the first commit from review, as it is already covered in #2388