This removes the usage of `syscall.Mkfifo` which was previously used and
won't work on Windows systems and opens the door for other processes on
the computer to interfere in the rtmp stream (dumping bad content in the
fifo, removing the file, blocking the file in offline status).
Instead, this patch introduces an `io.Pipe` which pipes the RTMP stream
to the ffmpeg command while staying in Owncast.
Further links:
* ffmpeg on using `pipe:0` as an input: https://ffmpeg.org/ffmpeg-protocols.html#pipe
* Fix rtmp secret validation to allow `/` (#1069)
* add negative test cases for stuff before /live/
* simplify since Url.Path is already stripping the host
This means that we can simplify the code and make it much clearer.
Removes the tests that checked for the host and stuff between the host and /live/.
