HPCC-27646 Create theme editor

[jeclrsg]

Type of change:

• This change is a bug fix (non-breaking change which fixes an issue).
• This change is a new feature (non-breaking change which adds functionality).
• This change improves the code (refactor or other change that does not change the functionality)
• This change fixes warnings (the fix does not alter the functionality or the generated code)
• This change is a breaking change (fix or feature that will cause existing behavior to change).
• This change alters the query API (existing queries will have to be recompiled)

Checklist:

• My code follows the code style of this project.
  • My code does not create any new warnings from compiler, build system, or lint.
• The commit message is properly formatted and free of typos.
  • The commit message title makes sense in a changelog, by itself.
  • The commit is signed.
• My change requires a change to the documentation.
  • I have updated the documentation accordingly, or...
  • I have created a JIRA ticket to update the documentation.
  • Any new interfaces or exported functions are appropriately commented.
• I have read the CONTRIBUTORS document.
• The change has been fully tested:
  • I have added tests to cover my changes.
  • All new and existing tests passed.
  • I have checked that this change does not introduce memory leaks.
  • I have used Valgrind or similar tools to check for potential issues.
• I have given due consideration to all of the following potential concerns:
  • Scalability
  • Performance
  • Security
  • Thread-safety
  • Cloud-compatibility
  • Premature optimization
  • Existing deployed queries will not be broken
  • This change fixes the problem, not just the symptom
  • The target branch of this pull request is appropriate for such a change.
• There are no similar instances of the same problem that should be addressed
  • I have addressed them here
  • I have raised JIRA issues to address them separately
• This is a user interface / front-end modification
  • I have tested my changes in multiple modern browsers
  • The component(s) render as expected

Smoketest:

• Send notifications about my Pull Request position in Smoketest queue.
• Test my draft Pull Request.

Testing:

[jeclrsg]

@GordonSmith Reminder that everything in src-react/util/theme/... was just pulled in from the fluentui ThemeDesigner package, as all of that palette generation utility functionality wasn't exported.

[jeclrsg]

This could possibly be removed from the PR. It was in the branch that I had started several months ago, and so I just left it in. I thought I remembered these being used elsewhere, Icons or warning labels or something. But I can't find any references.