Crash fix plus various improvements

Upgrading to version 1.0.3 causes a `NullPointerException` for existing jobs, unless you go into the job config and explicitly enable/disable the "Use new URL" option.

This pull request fixes that, makes reading the configuration a bit more robust in general, and adds internationalisation, so that all texts in the plugin can be translated.

If you're ok with it, I have commit access and could merge this and release a new plugin version myself.