[Improvement] On HTTP connection, make it clear ConvertX won't work without the proper environment variable set.

[nicktheone]

Finally got around to setting up ConvertX and I had to bash my had for a few minutes on why it would start, allow me to register an account and even select my file to convert and the desired output format but when it came to actually converting the UI wouldn't let me click on the "Convert" button. It finally dawned on me that maybe it was because I'm using HTTP only on my local network and I had not set the appropriate variable when writing my compose file.

I think it'd be beneficial to either completely block any HTTP connection (with a clear error message) in case the appropriate environment variable isn't set HTTP_ALLOWED=true or, at least, make it clear that despite being able to access the web UI you won't be converting anything and that's why the button was useless.

[C4illin]

Yes definitely almost all bugs reported for this project is people forgetting to set HTTP_ALLOWED=true. I even made the mistake myself a week ago