Fix dark mode for metadata UI

[ajbozarth]

This is a follow up to #1327 fixing various bugs as well as adding
dark mode support.

Dark Mode support for material-ui has now been abstracted to a new
ui-component and MetadataEditor has been updated to use it.

Developer's Certificate of Origin 1.1

   By making a contribution to this project, I certify that:

   (a) The contribution was created in whole or in part by me and I
       have the right to submit it under the Apache License 2.0; or

   (b) The contribution is based upon previous work that, to the best
       of my knowledge, is covered under an appropriate open source
       license and I have the right under that license to submit that
       work with modifications, whether created in whole or in part
       by me, under the same open source license (unless I am
       permitted to submit under a different license), as indicated
       in the file; or

   (c) The contribution was provided directly to me by some other
       person who certified (a), (b) or (c) and I have not modified
       it.

   (d) I understand and agree that this project and the contribution
       are public and that a record of the contribution (including all
       personal information I submit with it, including my sign-off) is
       maintained indefinitely and may be redistributed consistent with
       this project or the open source license(s) involved.

[elyra-bot]

Thanks for making a pull request to Elyra!

To try out this branch on binder, follow this link:

[ajbozarth]

Note those those I tagged for review, this PR touches a lot of code from #1293 if you could make sure I haven't broken anything from that PR (I spent all afternoon making sure I didn't but odd are I missed something)

[ajbozarth]

For reasons I am still unsure of, this PR makes the code editor in the MetadataEditor (used by code snippets) not load. This is (obviously) causing the tests to fail. I will look into why this is happening tomorrow

[ajbozarth]

I have made some changes to match @bourdakos1 comments above but I was dealing with rendering race conditions all afternoon. @bourdakos1 I'll ping you tomorrow and see if we can hop on a Webex and you can help me try to figure it out.

If I cant figure it out by early afternoon (pacific time) I’m going to partially roll back my work so that the part of the PR that fixes dark mode for the drop down can at least be merged since that part already works fine. That will unblock the 2.1 release, and then we can deal with moving the MetadataEditor to use the JpThemeProvider component in a follow up PR since that’s where the race condition is happening.

[bourdakos1]

This looks good to me

[ajbozarth]

@lresende once tests pass this should be good to merge (unless you want more reviews first)