Add maintenance page

• new maintenance page on indiehosters.net/maintenance/
• customisation on the fly with url parameters
• parameters available : what (which tool or instance is impacted?), info (link to forum/post about the maintenance), egg (link to an easter egg!), when (specifiy when it will be back. eg: 17h30), email (if =1, will show support email)
• exemple = https://indiehosters.net/maintenance/?what=RocketChat&when=17h&info=https://forum.indie.host/&egg=https://www.youtube.com/watch?v=dQw4w9WgXcQ&email=1
• tested with no (or partial) url parameters
• tested on desktop and mobile
• tested with js disabled