Add maintenance page

What does this MR do?

When doing maintenance we need a nice page!

This works in combination with some nginx configuration to check if a file is in place, then serve up a 503 error with the nice page.

The page is designed to be self contained, so I made it have data uris for the font and logo. Maybe it's overkill, as we still serve up static images...

The nginx configuration that needs to be added would be:

server {
  location / {
    # this needs adding inside every location block we want to serve the page
    # from such as a ~ \.php one...
    # the path needs adapting...
    if (-f /app/maintenance_mode_on) {
     return 503;
    }
  }
  error_page 503 @maintenance;
  location @maintenance {
    try_files /maintenance.html =404;
  }
}

To enter maintenance mode:

touch maintenance_mode_on

to leave maintenance mode:

rm maintenance_mode_on

How confident are you it won't break things if deployed?

Should be ok :)

Links to related issues

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• joined #foodsharing-beta channel at https://slackin.yunity.org
• added an entry to CHANGELOG.md (description, merge request link, username(s))