Add a diagnostic mode

[Maxou44]

Add a /api/diagnostic route on load balancer and make the following checks:

• Check HTTP/HTTPS consistencies between Load balancer and transcoder (if transocder using http and loadbalancer https, redirection fails)
• Check the number of transcoders registered (if there is no transcoder, start a stream fails directly)
• Check that transcoder can be called (if the load balancer can't fetch the / of the transcoder, there is an issue)
• Try to SELECT into plex database (else downlaod and forward will fail)
• Check if we already received data from fake ffmpeg
• Check if we can contact plex over http (if the load balancer can't talk with plex, streams will fails)

[Maxou44]

It could also be nice to make debug on the / of each transcoder to see if the transcoder is correctly installed and add some stats

[Maxou44]

We added a homepage on the transcoder part, it's a good beginning ☺️