fix: move debug mode to config

[dvikan]

Make it possible to toggle the debug mode with a config change. Preserve old DEBUG file if present.
Not moving the debug whitelisting to config. I suspect it's rarely used. Maybe wrong on that.
My motivation was to prepare separation between debug mode and various cache behavior due to future features.

[dvikan]

Perhaps a better config name is debug_mode or enable_debug_mode.

[em92]

Not moving the debug whitelisting to config. I suspect it's rarely used.

It is used. In my instance for example. curl https://feed.eugenemolotov.ru/DEBUG
Debug whitelisting is very convenient, so it should be saved.

[dvikan]

Add debug_mode_whitelist to config after feedback from @em92

[em92]

Previously debug mode messaged differed, when comparing debug mode for some IP address and debug mode for everyone
https://i.imgur.com/6Gri9S4.png
https://i.imgur.com/BBWEuPZ.png

I sugget to leave same difference between messages. Including background colors.

[dvikan]

@em92 Fixed.

[em92]

Another thing I payed attention - text looks better when using "text-align: center" like incurrent master.
PR: https://imgur.com/C2cheea.png
Current master: https://imgur.com/5hmWQha.png

[dvikan]

Sure. Agreed.