Improve download scripts

[kevinunger]

The download scripts in scripts/ do not do error handling, are a bit redundant and don't show the download progress. Also there needs to be scripts for windows and linux.
This PR incudes a download script that:

• works cross-platform
• prints the progress and connection info
• skips already downloaded files.
• all checkpoints names and urls are defined in a .json

Output while downloading:

C:\Users\kevin\Gits\forks\DragGAN>python scripts/download_model.py
checkpoints\stylegan2_lions_512_pytorch.pkl already exists but size does not match. Redownloading.
100%|█████████████████████████████| 364M/364M [01:04<00:00, 5.68MiB/s]
Downloading stylegan2_dogs_1024_pytorch.pkl from https://storage.googleapis.com/self-distilled-stylegan/dogs_1024_pytorch.pkl
  4%|█                           | 14.6M/382M [00:02<01:03, 5.79MiB/s]