Configure the Production environment

[dpordomingo]

required by https://github.com/src-d/code-annotation/issues/215

This PR prepares the Prod env.

Changes done:

• this application now is deployed into Staging and Production following our continuous-delivery guide,
  • every new tag will trigger a deploy into Production,
• gaTrackingID must be passed by drone.io when deploying
  • Staging will have its own GA_TRACKING_ID for testing purposes
• Staging will no longer be accessible for no @src-d/applications members

Other changes that should be done by @src-d/infrastructure just after merging this PR:

• https://github.com/src-d/issues-infrastructure/issues/152

[dpordomingo]

@bzz your documentation suggestions were added by https://github.com/src-d/code-annotation/pull/218/commits/14b4c49909aedb3c03332db12ee7d72981804058 Many thanks for proposing them!