Add maintenance page to Baremetal deploy

[cannikin]

If you've got a big enough change sometimes you need to take the whole site down temporarily while database migrations are running, or something needs to be re-configured on the site.

Let's add the ability to put a maintenance page up on the site so that all requests return that page instead of the index.

We create a new maintenance.html in the root of your app and you can put whatever you want in there (static HTML or a React app launching page, if you know what you're doing). At the time the command is run we have the web server serve the maintenance page instead of the regular index page. I'm thinking we just rename the files. When you run the maintenance:down command it renames everything back the way it was.

Right now I'm thinking syntax like:

yarn rw deploy baremetal --maintenance:up
yarn rw deploy baremetal --maintenance:down