Add Settings page

[KokkieH]

This adds a settings page for the user to input credentials for the quotes API that will be used to generate the quotes.

To test this pull request

• Check out this PR and copy the folder into your test site's /plugins folder (if you're using Docker and wp-env, navigate to the plugin folder in the command line and run wp-env start)

• Log into WP-Admin and activate the plugin

• Under Settings should be a new menu item, Random Quote SPA:

  

• Visit Settings page and ensure it loads:

  

• Enter a username and API key and save. Navigate away and return to settings page and ensure entered credentials are still visible

• Change or completely remove credentials and repeat previous step to confirm the changes saved

[KokkieH]

Going ahead and merging this, as everything works as expected.