Add web installer

[martbock]

It would be really cool to have a web installer like one of the typical CMS's.

It should do the following things:

• Check all server requirements (PHP version, needed extensions, etc.)
• Check read/write access to the essential folders and files
• Check that Laravel does not have write access to the .env file (because of the integration shell commands)
• Provide a form to generate a new .env file
• Generate an application key and show the compiled .env file template for the user to copy and place in the project directory
• Generate a send_only mailbox for Laravel's mailing driver
• Supervisor Configuration for running queue worker