Honor user's browser language (#558)

Honor user's browser language (#558)

Previously we would attempt to satisfy the user's first language
preference, immediately falling back to english if that was not
possible. Now, we will get the best match of the user's preferred
languages.

This requires storing the available locales on app startup, so we
have mg_globals.available_locales ready to compare them against the
list of preferred user languages.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>