A locale is considered usable if it is both (a) available in the
filesystem and (b) configured by the administrator in
SDConfig.SUPPORTED_LANGUAGES.  Once we've determined which configured
locales are actually usable, we:

1. warn if a configured locale is not available;

2. fall back to the hard-coded FALLBACK_LOCALE ("en_US") if
   SDConfig.DEFAULT_LOCALE is not usable; and

3. error out if neither the default nor the fallback locale is usable.

We can't iterate directly over USABLE_LOCALES: Set(Locale) because
Locales aren't sortable.  Instead, we parse config.SUPPORTED_LOCALES in
order and skip those that aren't in USABLE_LOCALES.

… to disambiguate SUPPORTED_LOCALES

thread: #6406 (comment)

This is a safe assumption except in the corner case where (e.g.):

1. A user begins a session and selects language L.

2. The Application Server upgrades to a version of securedrop-app-code
   that removes support for language L.

3. The user resumes their session, which still has language L selected.

But, since this case has come up in testing[1], best to check for it.

[1]: #6406 (comment)

As of 9c11950, get_locale() handled user/agent-provided locale
preferences and deferred to resolve_fallback_locale() if none
could be satisfied.  Here we build a single list of locales,
beginning with user/agent preferences and ending with the server-side
{DEFAULT_FALLBACK}_LOCALEs, and call babel.core.negotiate_locale()
once to negotiate one (if possible).