Metadata in UNGA Slides not yet changed

[ChristophLeonhardt]

The UNGA Slides for now are first and foremost a translated version of the original UCSSR slides which were published mainly in October 2018. In addition, the English UNGA corpus was used. Since the originality is limited thus far, I did not change authorship and date for now, but added a "english_version" element in the yaml header of the R Markdown files to indicate that I changed the slides. I am not sure if that's the best way. I did not render the HTML yet so there is still ample opportunity to change this later on, I guess.