Make Pulse deployable to cloud.gov

[konklone]

The site can now be run on cloud.gov:

• Production: https://pulse.fr.cloud.gov/https/domains/
• Staging: https://pulse-staging.fr.cloud.gov/https/domains/

We can update the DNS, in collaboration with the Cloud.gov team and the OMB MAX team, any time.

Note that the data will not update automatically anymore. We will have to manually kick off scans each week, and re-freeze the updated data into the repo and redeploy. The right place for running scans long-term in our environment is not clear, though I have copied the current scanning workhorse server AMI into our new AWS sandbox environment, and I expect to use that server for scanning for the time being.

This PR does a few things:

• Serves the app using waitress in production, if PULSE_ENV is set to production.
• Adds a cloud.gov manifest with a production and staging instance, both using the Python buildpack and a Python runtime of 3.5.2.
• Causes the /accessibility/ URLs to return a 404 based on the set environment variables, rather than the code itself. This means we don't have to maintain a weird code split between branches anymore, which will simplify future feature work considerably.
• Since we don't have to have master diverge from production anymore, this merges in all commits from the master branch that aren't in production.
• In order to freeze the data effectively into the repository and create consistent deployments, data/db.json is now committed to version control. It's not pretty when diffed, and even more annoyingly, it seems to change just by running the app locally. But this is by far the easiest way to freeze the data into the repository, and necessitates no other architectural changes.

[konklone]

You can see the env-var-driven hiding of accessibility endpoints in the cloud.gov deployment of the production and staging apps here:

• Production (404): https://pulse.fr.cloud.gov/accessibility/domains/
• Staging (200): https://pulse-staging.fr.cloud.gov/accessibility/domains/

[konklone]

Since the merge of master makes the diff difficult, here are the commits that are new and unique to this pull request:

• Getting the app to run on cloud.gov: https://github.com/18F/pulse/pull/586/commits/d963dd22edbf6492bdbcd93b3cb56a76cc8d422f
• Adding the database to source control: https://github.com/18F/pulse/pull/586/commits/7324110c1544f10b422b76221108bf49352b8971
• Defining a staging app for cloud.gov and hiding a11y endpoints: https://github.com/18F/pulse/pull/586/commits/f4cfe9b11be66f22116f6d306ba54e06e60c4bb9