Run streamlit using user supplied checkpoint

[csteinmetz1]

Had to move webapp.py to the root directory so that it could access the modules in lightning directory. I test this with my own pretrained hybrid demucs and it works. Would be good if someone else can test it out.

Also adding png version of the logo for compatibility with streamlit.

[yoyolicoris]

Thanks for the PR. I left some comments.
I haven't tried your script yet; I will try it when I have time.

[yoyolicoris]

sources ordering problem raised in #20