Bug 42305: Add script to combine translation files across versions.

tools/torbrowser/l10n/combine-translation-versions.py

tools/torbrowser/l10n/combine/__init__.py

tools/torbrowser/l10n/combine/combine.py

tools/torbrowser/l10n/combine/tests/README

tools/torbrowser/l10n/combine/tests/__init__.py

tools/torbrowser/l10n/combine/tests/test_android.py

tools/torbrowser/l10n/combine/tests/test_dtd.py

tools/torbrowser/l10n/combine/tests/test_fluent.py

tools/torbrowser/l10n/combine/tests/test_properties.py