Add Purple checkout maintenance mode option

[danieldaquino]

This adds a purple checkout "maintenance mode" option.

To activate it, simply make a route called /purple-checkout-maintenance available (returns HTTP 200).

In a static build, this can be done by creating a file called purple-checkout-maintenance at the root of wherever the static built files are being hosted:

touch /path/to/static/files/purple-checkout-maintenance

When present, the checkout page will be replaced with a maintenance message.

How it looks like

Testing

Website: 4aeb94d98eef34e1bd9dc0accc8ceefca393407c
Setup: Running on npm run dev
Steps:

1. Go to purple checkout, see if shows up. PASS
2. touch public/purple-checkout-maintenance on the local repo
3. Refresh page, make sure maintenance message appears. PASS

Other notes

This PR is built on top of e9a9ffee71317c51fe6c8b9c8c32ff18bd006eea, which does not include changes from damus-io/notedeck#330, and this is on purpose as that is still only deployed in staging and this PR is a "hot-fix" of sorts.