This implements the configuration mechanism I described in #2621. The heavy lifting is delegated to [node-config](https://github.com/lorenwest/node-config) with a minor assist from [dotenv](https://github.com/motdotla/dotenv).
`private/secret.json` has been replaced with environment variables and/or `config/local.yml`. See `doc/server-secrets.md`.
* don't use a libraries.io token for bower integration
The libraries.io docs claim you need to be authenticated
to make any API request: https://libraries.io/api#authentication
In practice we can call https://libraries.io/api/bower/jquery
just fine with no token and based on chucking a load of
requests at it and examining the `x-ratelimit-remaining`
headers you actually seem to get a better limit with no
authentication.
All of our libraries.io badges in `services/librariesio`
seem to have been running fine with no token for some time.
* change jira auth settings to jira_user, jira_pass
All the other services use servicename_user, servicename_pass
This switches JIRA to use that convention by preference
but supports _username and _password for legacy users.
* add docs for server secrets
* add danger rule for server-secrets.md
this rule prompts users to update server-secrets.md
if 'serverSecrets' is in the diff
