Avoid geocoding when seeding the db during development

[DeeDeeG]

Context

See this comment (bullet point number 3) by @btyy77c that brought this to my attention: https://github.com/RefugeRestrooms/refugerestrooms/pull/508#issuecomment-427796840

Scope and Proposal

Not too hard, I hope.

• Add an environment variable (SEEDING_DONT_GEOCODE) or similar.
  • Probably set this in setup/entry before rails db:setup, then unset it right after that.
• Modify our controllers or models to check this environment variable and not geocode when environment variable (SEEDING_DONT_GEOCODE) is set.
  • Probably here: https://github.com/RefugeRestrooms/refugerestrooms/blob/develop/app/models/restroom.rb#L80
  • Or else here?: https://github.com/RefugeRestrooms/refugerestrooms/blob/develop/restrooms_controller.rb

Impact

• Stops us from using 21 restroom submissions' worth of quota every time we bring up the Docker image.
• Eliminates error message:
  • Google Geocoding API error: over query limit.