Onboarding and settings refactor to React

[iamdharmesh]

Description of the Change

The PR refactors the settings and onboarding to a React-based implementation and deprecates the existing settings panel. The existing settings panel is deprecated but can still be enabled using the classifai_use_legacy_settings_panel filter.

Closes #502
Closes #449

How to test the Change

Settings:

1. Go to Tools > ClassifAI.
2. Ensure the settings page loads correctly and that navigation works as expected.
3. Update feature settings, such as enabling/disabling features or changing/configuring providers, and verify that everything functions as expected.
4. Existing settings: Configure the ClassifAI plugin using the develop branch, then switch to this PR branch. Confirm that no settings are lost and that all data is correctly reflected in the new settings page.
5. Use Legacy Settings: Add the following code to the theme’s functions.php file or plugin: add_filter( 'classifai_use_legacy_settings_panel', '__return_true' );. Verify that the old settings page loads and works as expected.

OnBoarding

1. Deactivate the ClassifAI plugin and activate it again.
2. You will see a notice to start the ClassifAI setup. Click the "Start Setup" button.
3. Follow the setup process and ensure everything works as expected.
4. Existing settings: Visit the setup page from the settings page and verify that it correctly reflects the existing configured features, with the option to modify the settings accordingly.
5. Use Legacy Settings: Add the following code to the theme’s functions.php file or plugin: add_filter( 'classifai_use_legacy_settings_panel', '__return_true' );. Verify that the old onboarding/setup page loads and works as expected.

Add additional feature fields

1. Add a new feature to the plugin by following the guide here.
2. Follow step 4 to add additional feature fields to the settings and validate that everything works as expected.

Add custom Provider fields

1. Add a custom provider to any feature by following the guide here.
2. Add the required provider fields to the settings by following step 5 in the guide and validate that everything works as expected.

General
Run a full regression test to ensure everything is working as expected.

Changelog Entry

Added - New React-based settings and onboarding process.
Deprecated - The existing settings panel.

Credits

Props @jeffpaul @dkotter @iamdharmesh @Sidsector9

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[Sidsector9]

@iamdharmesh below, I've checked the "Editor" checkbox under "Allowed roles". Ideally it should have just re-rendered the area pointed by the arrow, but instead the whole App re-renders. We can consider improving this.

[Sidsector9]

Preventing access of state at the root App may help avoid re-rendering. I am working on and likely to fix this.

[Sidsector9]

Improved areas where re-rendering wasn't necessary. I'm trying to further improve this.

[Sidsector9]

Note to self:

Find out a way to make this easier so that 3rd-party devs don't have to write Fill.
Maybe try exporting a component on the window object which can be used to inject custom Provider settings.