Remove deprecated basic settings; centralize validation declaration

[joshcanhelp]

Changes

• Remove deprecated method WP_Auth0_Admin_Basic::render_auth0_app_token()
• Remove deprecated method WP_Auth0_Admin_Basic::render_allow_signup_regular_multisite()
• Remove deprecated method WP_Auth0_Admin_Basic::render_allow_signup_regular()
• Remove deprecated method WP_Auth0_Admin_Basic::render_basic_description()
• Remove method WP_Auth0_Admin_Basic::render_auth0_app_token() and replace with wp_auth0_ajax_delete_cache_transient() to handle JWKS delete cache AJAX action.
• Remove the WP_Auth0_Admin_Basic::BASIC_DESCRIPTION class constant
• Remove the WP_Auth0_Admin_Features::FEATURES_DESCRIPTION class constant
• Moved $_description and $actions_middlewares definition to main class

Testing

• This change adds unit test coverage
• This change has been tested on WP 5.2.2

[luisrudge]

Previously, it just called exit. Why are you returning json_success now? https://github.com/auth0/wp-auth0/pull/722/files#diff-729ab756a0f851fef2f036fa7ca87b13L351

[joshcanhelp]

wp_send_json_success() is a core WP function that returns a successful response, then calls exit (or die which is the same). Just provides a little more feedback to the calling script.