Configure `Metrics/BlockLength` with sensible Rails defaults

[jgarber623-cargosense]

I've found a handful of blocks that are necessarily long and not easily avoidable:

• Rails.application.configure
• Rails.routes.draw (and namespace within the routes file)
• reversible in database migrations
• task in a *.rake file

It's not terribly helpful to have RuboCop shout about those, so I'm proposing we add the following to our default metrics configuration here to avoid the noise:

Metrics/BlockLength:
  AllowedMethods:
    - configure
    - draw
    - namespace
    - reversible
    - task