Provide an unobtrustive mechanism for supplying secrets.

Consolidate the secrets files used by `dev_web.yml` and `site.yml` into a single file. Keep production secrets separated in an `ansible-vault`'d `secrets.prod.yml` file, and use it for `script/deploy`.

1. On a fresh `git clone`, anyone should be able to run `vagrant up` successfully. The node.js backend will be configured with empty values, so form submissions won't work.
2. On a fresh `git clone`, production users (@ycombinator) should be able to `script/deploy` successfully, using the Real Credentials, by providing the vault password.
3. In either situation, users should be able to *override* the default credentials by providing a (`.gitignore`'d) `secrets.yml`. This will let @kenperkins (for example) do development work on the node.js backend, either on Vagrant or a real Cloud Server, without having to worry about committing *his* credentials to GitHub.

Fixes #395.