[MuiThemeProvider] Add a muiThemeProviderFactory

[viotti]

I took the liberty of making the rebase of #6900 whilst preserving the core changes of 8ab5299. Hope you guys don't mind. The only conflict to solve was related to the theme leaving the child context of MuiThemeProvider. I also had to make a couple of adjustments to satisfy prettier.

• PR has tests / docs demo, and is linted.

I ran npm run test, but the commit has no new test cases.

• Commit and PR titles begin with [ComponentName], and are in imperative form: "[Component] Fix leaky abstraction".
• Description explains the issue / use-case resolved, and auto-closes the related issue(s) (http://tr.im/vFqem).

Closes #6900
Closes #6877

[oliviertassinari]

Thanks :)

[SukantGujar]

Thank you so much. I have been occupied by work over the last few weeks and couldn't get to this at all. Moments like this make one really appreciate the efforts OSS maintainers put into their work. Its so challenging to be able to balance between job and other commitments.

[oliviertassinari]

@SukantGujar Simply raising that point was valuable, thanks 👍 .

[mbrookes]

7000th PR merged!

Edit: no - wait, that's wrong. PRs and issues share the same sequence. Oh well:

PR# 7000 merged!