* Add config option MediaUploadPath
MediaDownloadPath can be used instead of MediaServerUpload, for when your
webserver is on the same system as matterbridge and matterbridge has
write access to the serve dir.
* Limit length of hash in MediaServer urls to 8chars
Full SHA256 is unnecessary for uniqueness.
Also; if a file has the same first 8 charachters of the SHA256 hash,
it's still not a problem, as long as the filename is not the same.
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