Use Python logo for IPython kernel

Instead of IP[y]: logo

Closes gh-7189
Fill full logo space with Python + sign
Probably don't need text styles on an img tag
Enlarge Jupyter logo to 28px height
Make Python logo a 28 (56) pixel square in a 32 (64) pixel image
Merge pull request #7199 from takluyver/python-logo

Use Python logo for IPython kernel