Testing application first use experience

@vkrause, I presume that that’s meant to be XDG_CONFIG_HOME?

A little disappointing that this isn’t syntax highlighted, despite it being so at its origin, considering Markdown makes it fairly easy:

```py
import os
import subprocess
import sys
import tempfile
xdgHome = tempfile.TemporaryDirectory(prefix='testing-')
for d in ['CACHE', 'CONFIG', 'DATA', 'STATE']:
    os.mkdir(os.path.join(xdgHome.name, d))
    os.environ[f"XDG_{d}_HOME"] = os.path.join(xdgHome.name, d)
os.environ['AKONADI_INSTANCE'] = 'testing'
subprocess.call(sys.argv[1:])
subprocess.call(['akonadictl', '--instance', 'testing', 'stop', '--wait'])
xdgHome.cleanup()
```

I don’t appear to be able to force it either, so I presume it’s a deficiency of this Discourse instance’s rendering plugin or configuration choices, rather than the cross-poster omitting it?