Some MTAs will reject unverified sender addresses, and we currently have
no valid setup to send from `@translate.tails.boum.org`. Let's use
`@lizard.tails.boum.org` instead (at least it's an MX of itself)!

Example of the problem:

    (host eugeni.torproject.org[94.130.28.202] said: 450 4.1.7
    <root@translate.tails.boum.org>: Sender address rejected: unverified
    address: host lizard.tails.boum.org[198.252.153.59] said: 554 5.7.1
    <root@translate.tails.boum.org>: Relay access denied (in reply to RCPT
    TO command) (in reply to RCPT TO command))

I need to temporarily disable cron.sh for #17005.

The workaround introduced in 6b292cc0
for putting the website in maintenance reports "Under maintenance" for
any 503 and misleads people when the 503 is actually caused by some
other reason.

This commit reverts that behavior.

This reverts commit 4353d8bf.

See: https://salsa.debian.org/tails-team/gitlab-migration/-/issues/58

https://salsa.debian.org/tails-team/gitlab-migration/-/issues/48

Neither Weblate nor Django have built-in support for putting the website
in maintenance mode, so I'm adding the possibility of returning a 503
HTTP status code by touching a ".maintenance" file in Weblate's document
root.