rework and fix handling of JS translations, re #5

Makefile

cli/Commands/Translations/VueGettextSplitTranslations.php

cli/studip

locale/compileTranslation

locale/de/LC_MESSAGES/js-resources.json

locale/en/LC_MESSAGES/js-resources.json

locale/en/LC_MESSAGES/js-resources.po

locale/en/LC_MESSAGES/studip.mo.old

locale/en/LC_MESSAGES/studip.po.old

locale/extractTranslationStrings

resources/assets/javascripts/chunks/vue.js

resources/assets/javascripts/lib/gettext.js

resources/locales/en_GB.json