Application may be long to start (e.g. a Docker app may have to download images first,
and even without the downloading, the starting could be long), which may lead to UI
blocking or bridge time out.
To prevent that, `start` is now returning immediately, and 2 new signals are used to
indicate when the application is started, of if something wrong happened.
`start` now returns initial app data, including exposed data without the computed exposed
data. The computed data must be retrieved after the app has been started.
This file is used to set `USE_X_FORWARDED_HOST`, which is necessary to make Weblate work
correctly being an Apache (and potentially other HTTP servers) reverse proxy (otherwise we
have CSRF errors, cf. https://github.com/WeblateOrg/weblate/issues/1912).
when `email_from` was not set in `sat.conf`, no value was set for `WEBLATE_SERVER_EMAIL`
and `WEBLATE_DEFAULT_FROM_EMAIL`. These environment variables are needed for Weblate to be
launched, so default value are now used (the same as the one from official Weblate Docker
image).
