Unsafe chat names when downloading media

[gumblex]

Some chat names, eg. "a/b/c", when using with
MediaFilenameFmt = usermedia/{name}-{context_id}/{type}-{filename}
can create folders like "usermedia/a/b/c--123456/document-filename.123.txt"

This is not intended and unsafe (like a/../../b), and also have problems with other filesystem filename restrictions. Please really sanitize the chat name, sender name and file names. For example, replace " * / : < > ? \ | characters with _.

[Lonami]

#99 also noticed this issue, although the PR is unrelated.

[gumblex]

#99 only fixes filename, not chat name and sender name