Add new BERT models for German (cased and uncased)
Hi,

this PR adds new BERT models for German (both cased and uncased) from @dbmdz.

Details can be found in [this repository](https://github.com/dbmdz/german-bert).

Tasks:

* [x] Models are stored on S3, only permissions need to be adjusted by @julien-c 
Great, ok all the models should be public. Merging this now.
Awesome work @stefan-it!