Web migrate toga css

[moondial-pal]

Finalized migration to insert based web deployment

Removed deprecated static/toga.css file and added ...inserts/static/css/breifcase.css~css to accommodate insert based web deployment.

Fixes #3822

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[moondial-pal]

Not sure if this does the trick but did the following to validate.
Rebuilt the toga-web wheel and verified the insert is included using:

python -m zipfile -l dist toga_web-0.5.3.dev177+g87ddf50f1-py3-none-any.whl | grep briefcase.css~css

[freakboy3742]

Two comments:

1. As of beeware/briefcase-web-static-template#22, the style file is named style.css, not briefcase.css
2. There's a timed element to this PR; we can't land it for at least 6 months. This is a backwards incompatible change for Briefcase, so we need to release a version of Briefcase that has the new web media handling, and then wait for that code to become widely deployed.

[moondial-pal]

Ah, thanks for the review and the reference/link.
I'll make the necessary change.
I did see the instruction on ~6 months - but since I'm still getting my feet wet contributing to this project, I wanted to submit the PR anyway so the work was visible and could be reviewed early. Knew I would be bound to fumble something...and sure enough🙂.

[moondial-pal]

Made the requested change to the filename.
Let me know if there is anything else.
Now we let the PR sit on ice for 6 months?

[freakboy3742]

Yes - I think this is all that is needed; now we wait :-)