Use dotenv-rails instead of straight dotenv

dotenv-rails uses .env.#{Rails.env} if available, which allows us to
configure environment-specific variables. As such we now include the
base .env file with the repo, and instead ignore local development
configuration.
Switch to ENV variable style .env file
Configure Supermarket with ENV variables

The various components that were once configured with magiconf but which
are now configured with ENV variables:

  - StatsD
  - The CLA Location
  - NewRelic
  - SMTP
  - PubSubHubbub secrets
  - OmniAuth
  - Octokit
  - Raven
  - Mailer URL
  - CLA notification recipient
  - Default sender address in ActionMailer
  - the app's secret key base
  - Segment.IO key
  - Paperclip S3 settings
  - the ICLA/CCLA version
  - the PubSubHubbub callback URL
  - Curry's success label
Remove StatsD

We never ended up tracking any metrics with it. If/when we do, this
commit should revert cleanly (Gemfile and Gemfile.lock notwithstanding)
Remove non-essential variables from .env

SMTP, Sentry, S3, and New Relic are all handy to tune in production, but
just add noise in development and test. For Sentry and SMTP, fake
settings can cause unusual errors, as well.

Finally, CHEF_OAUTH2_SITE is unused, so we just remove it.
Expand CONFIGURING.md with all variables and dotenv info
Start working on the contributors table specs.
Relax feature spec assertions
Test the presence of Supermarket's repositories
Test presence of agreements in contribute section
Tighten scope of contributors feature specs
Add user id to server side segment io events

Server side segment io events now include the current users id if a user
is currently logged in otherwise it is tracked as anonymous.
Specify path in transfer ownership form

When in the cookbook version view the transfer ownership form is unable
to resolve a URL so this specifies the path instead.
Give .env.production precedence over .env
Add a note to an unclear Curry spec
Organization contributors are known commit authors

Previously, only users who had signed either an ICLA or a CCLA were
considered known and thus were the only users who Curry considered
"verified." This was incorrect; users who were invited to contribute on
behalf of an Organization are also verified.
Verify a new contributor's Pull Requests.
Rename signed_cla to authorized_to_contribute on Curry::CommitAuthor.
Rename commit author import method names

These new method names reflect a better understanding of the
requirements surrounding which commit authors are authorized to
contribute to projects watched by Supermarket.
Rename commit author import classes for brevity