translate content files earlier in build, fix #30

this moves the compilation of .po files and 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