refactor: don't use anchor link in form action

[thecristen]

Summary of changes

Asana Ticket: Trip planner 500 error

Rendering the trip planner results requires parsing the form's selections via query params. We had a bug where, when specific modes are requested, the selected modes were not being resolved as query params cleanly because of our use of an anchor link (we send users to a certain spot in the DOM by appending #plan_result_focus to the URL), so we'd end up with this as the mode:

%{
  "bus" => "true",
  "commuter_rail" => "true",
  "ferry" => "true#plan_result_focus",
  "subway" => "true"
}

I decided to take an alternate approach and abstain from using an anchor link in this case. Here we use a tiny bit of JS to scroll to the trip planner result. I wanted to use JS.dispatch (which supposedly works on dead views) but could not get it to work properly; perhaps that'll work better in more up-to-date Phoenix/LiveView versions.