Installer UX enhancements

[LukeTowers]

• Prepopulate the site URL value with the location of the current document
• Perform validation on each configuration step before proceeding to the next step
• Provide example path for the default sqlite path so that it's more obvious whether a relative path or an absolute path are expected (No longer necessary, SQLite DB will be setup in default location - the path is provided in the installer)
• Don't require all the "Admin" detail fields to be filled out (double check what WP requires in their installer)
• Nice to have: Animate the progress bar so that it seems like it's moving even when it's not, kind of like the Windows one
• View your install & View your backend should be links rather than buttons so that users can command click or otherwise open in new tabs if they want to be able to view both from the install page.
• Don't include unnecessary files (see the marketplace API for what files are considered unnecessary for new installs, immediately noticed the .github file)