Add new Torbutton file securityLevel.properties to the translations repo

legacy/trac#25658 (moved) brings a bunch of new strings to translate along which are organized in a new file, securityLevel.properties. We need to get that added to the translations repo to get the strings in all the needed locales.

Currently the code in Torbutton's import-translations.sh expects the branch to be torbutton-securityLevelproperties but that's easily adjustable if needed.