Surveys: Accept (lat,lon) of stops in responses

[aaronbrethorst]

Companion issue: OneBusAway/onebusaway-android#1259

This comes from our conversation with Devin Braun.

User Story

As a transit agency employee,
I want to see lat,lon values in survey responses,
so that I can visualize locations in my data analysis tool of choice

Acceptance criteria

• Add stop_latitude and stop_longitude columns to SurveyResponse (see below).
• Modify Api::V1::SurveyResponsesController#create so that stop_latitude and stop_longitude are accepted as params.
• Add this new data to the data rendered from survey_responses#index, both the HTML and CSV variants.

Migration

modify_table :survey_responses do |t|
  t.column :lat, :decimal, precision: 15, scale: 10
  t.column :lng, :decimal, precision: 15, scale: 10
end