Maintenance htaccess file to couple with https://gist.github.com/MatteoGgl/b2638d69806f866cbc3e5fce37747731

.htaccess

# MAINTENANCE-PAGE REDIRECT
<IfModule mod_rewrite.c>
     RewriteEngine on
     RewriteCond %{REMOTE_ADDR} !^123\.456\.789\.000
     RewriteCond %{REQUEST_URI} !/maintenance.html$ [NC]
     RewriteCond %{REQUEST_URI} !\.(jpe?g?|png|gif) [NC]
     RewriteRule .* /maintenance.html [R=302,L]
</IfModule>