Upgrade Ping CRM to latest version of Inertia.js

[reinink]

🎉 This is a big update to Ping CRM, which includes:

• Updated to the latest version of Inertia.
• Updated to the latest version of the Vue 2 adapter.
• Updated to the latest version the progress library.
• Updated to Laravel Mix 6.
• Updated to Tailwind CSS 2.0.
• Converted all forms to use the new form helper.
• Massively simplified pagination (thanks to some pagination updates in Laravel - see here and here).
• Updated all $page prop references to $page.props.
• Updated the menu component to use the new reactive $page.url property.
• Improved code formatting (using Prettier and ESLint).