Fix improve and update the tornado branding

[joegasewicz]

This updates the current logo by increasing the size of the file (it has been optimised for web) but maintains the color & design & font as closely as possible.

Also updates the README to include the logo in the header, similar to Flask & FastAPI's READMEs.

[bdarnell]

Why does the logo need to be almost 7 megapixels? I could see it doubling in size since display resolution has increased a bit since this logo file was
created in 2009, but that's just excessive (and would, I think, increase the size of our sdists by about 20%). I'm also not a fan of putting a blue circle behind the logo instead of letting it have its unique silhouette.

Adding the logo to the readme is fine, but we don't need a separate version of the file. The same logo should be used for both tornadoweb.org and the readme.

While you're here, does the gitter link work for you? I'm not able to sign in. We should probably remove that.

[joegasewicz]

Why does the logo need to be almost 7 megapixels? I could see it doubling in size since display resolution has increased a bit since this logo file was created in 2009, but that's just excessive (and would, I think, increase the size of our sdists by about 20%). I'm also not a fan of putting a blue circle behind the logo instead of letting it have its unique silhouette.

Adding the logo to the readme is fine, but we don't need a separate version of the file. The same logo should be used for both tornadoweb.org and the readme.

While you're here, does the gitter link work for you? I'm not able to sign in. We should probably remove that.

I'll make your suggested changes to the logo & update the PR tomorrow. No, i don't think the gitter link has worked for a while, I will also remove that then as part of this PR, thanks!