Add educational explanations to AmenityListing

[idreyn]

We have a component called AmenityListing that lists some good things about a given journey, e.g.:

• it uses electrified Regional Rail trains
• it runs through a segment of track that has had speed restrictions lifted
• it involves boarding at a level platform

It will be helpful to have corresponding bits of text about what each of these means and why they're important. Open to all sorts of ideas for how to display this.