Transit alert improvements

[graue]

Saw this alert on a test route and it made me think of 3 improvements that would be desirable:

• These alerts are confusing because they don't explicitly say which bus stop the alert applies to. (One applies to the 18th & Valencia stop, one applies to the 18th & Guerrero stop). We should have a way to expand details and explicitly show the scope of the alert, in this case the stop it applies to.
• These alerts are irrelevant to this trip because you don't board or alight at those stops. We should only show stop-specific alerts if you board or alight at, not just pass through, the stop. -> Only apply a stop-specific alert to legs that start or end at that stop bikehopper-web-app#23
• The alerts look goofy because there's a colon floating in space, because Muni provided only body text and no header text. Should omit the colon if one of body/header text is blank.