Remove Material UI ThemeProvider and Metadata header

[marthacryan]

Fixes #2951 - removes the use of ThemeProvider and other material UI components to remove material UI from imports.

Mostly maintains the same UI, but the main change is that it now uses the built-in context menu's from JupyterLab for the metadata header:

Fixes #2948
Fixes #2952

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[ajbozarth]

on a quick check there are a few things not changing when using dark mode:

category name & arrow icon:

icons for arrange buttons:

There also some issues in the metadata header but we're replacing that so no work needed there

[marthacryan]

@ajbozarth Thanks for catching those - I fixed the color of the palette categories but I checked and the icon colors are unrelated to my changes and if I'm remembering correctly, we did look into them and it was a canvas issue. We could make a separate issue for that but it's not actually straightforward to fix.

[ajbozarth]

I've push some updates including:

• I fixed the icon coloring I mentioned above (overrode it with css )
  
  
• I tinkered with the css classes on the header buttons, previously the context menu was added for both buttons so if you right clicked either the add or refresh it showed the add menu, I made it only show on the add button.
  
• I removed the final Material UI dependency from pipeline editor by replacing the SnackBar Alert with a ToastContainer provided by https://github.com/fkhadra/react-toastify (the library core will be using in 4.0)
  
  • Some change in toast functionality:
    • clicking outside the notification no longer clears it (this is not natively supported by toastify, but if it's an issue we can write our own handler to do it)
    • multiple toast notification stack rather than replace the current one

[ajbozarth]

@marthacryan aside from the minor fixed I added, your code changes look solid and the UX is great when run locally

[ptitzler]

LGTM!