For consistency this PR introduces using custom markdown and short upload:// URLs for video and audio uploads, rather than just treating them as links and relying on the oneboxer. The markdown syntax for videos is  and for audio it is .
This is achieved in discourse-markdown-it by modifying the rules for images in mardown-it via md.renderer.rules.image. We return HTML instead of the token when we encounter audio or video after | and the preview renders that HTML. Also when uploading an audio or video file we insert the relevant markdown into the composer.
When uploading an image file with dots in the filename we were splitting the string on dots and getting the last of the split items as the extension-less filename. However this did not work with filenames that have dots. We now just remove the extension using substr.
* instead of using encodeURIComponent in imageNameFromFileName,
we just replace the bad characters that we wanted to get rid
of in the first place where we introduced encodeURIComponent.
as per review
Meta: https://meta.discourse.org/t/image-name-has-20-in-file-name/134136
We were ending up with [file%20name](url) in the markdown preview, which looked weird and
affected the alt text. this is because we were calling encodeURIComponent, which has been left in place because this is a valid thing to do for some cases. (e.g. f674b9e)
