[Feature] Traffic Alerts

[pingzing]

For 1.1:

• [ ] Check that URL decoding handles äs and ös properly

For 1.2:

• [ ] Refactor things into an AlertsService that tracks alerts seen, and is a central repository for received alerts so multiple pages can navigate to the alerts page without needing outside context.
• [ ] Add an Alerts button to the header of the narrow TripResults page.

Show a traffic alerts button (if there are any) on the main page up in the header.

[pingzing]

Mostly done! One last thing to do is check and make sure we do proper URL decoding, so we handle äs and ös correctly.

That's low-prio enough that we can probably push it to 1.1.

[pingzing]

Something for 1.2: Refactor things into an AlertsService that tracks alerts seen, and is a central repository for received alerts so multiple pages can navigate to the alerts page without needing outside context.