this moves the translation of contents.lr files earlier in the build
process, in `on_before_build_all` instead of `on_before_build`, to
ensure that the build queue accounts for the alternative content files
(`contents+*.lr`) during the build process, instead of requiring
multiple (expensive) build runs
the caveat is that translated alternative contents files are now only
produced during a full build, which likely breaks "on-the-fly" creation
of those files when authoring using the Lektor web admin interface
