How to put a site in maintenance mode by non-admin Is there a module (or other way) to give permission to non-admin-users to put the site offline (and only that permission)?
There is a module for that!

Maintenance Mode Permissions


  Placing a site in or removing it from maintenance mode is allowed
  under a permission that has wide spread administrative access
  (administer site configuration). Sometimes you'd like to give
  permission to put the site in maintenance mode to someone but not give
  them complete access. This modules enables this case.