New menu List of Speakers: rename

[MSoeb]

Decription: The new menu in participants > 3-dot-menu > "List of speakers" needs a rename, because the name is to similiar to other list of speakers. This also affects the csv file name.

What should happen:
English wording
old menu name "List of speakers" -> new name "Contributions"
old csv file name lists_of_speakers.csv -> new name "contributions.csv"

German wording
old menu name "Redeliste" -> new name "Wortmeldungen"
old csv file name lists_of_speakers.csv -> new name "wortmeldungen.csv"

[MSoeb]

Important: Added information for german wording in the issue