Update logic for onboarding redirects

[wpscholar]

This PR utilizes the new Install Checker module to check if a site is a fresh WordPress install and updates the logic for redirects as follows:

• Always skip onboarding if user isn't an admin
• Allow manually disabling onboarding (via query param - sets DB option)
• Allow manually enabling onboarding (via query param - sets DB option)
• Check if onboarding is explicitly on or off (via DB option)
• If explicitly on, go ahead and trigger onboarding
• If explicitly off, bypass onboarding
• If not explicitly set, run through qualifiers
  • User hasn't completed or exited onboarding already
  • Site is fresh install (use module for this)
  • Coming soon is not enabled
• If onboarding redirect is triggered at any point, update the DB option to disable onboarding for future visits

[arunshenoy99]

Thanks @wpscholar, have included these as part of #373. Hence, closing out this PR.