docs: Add :discuss: shortcut for linking to forum threads
path: Support unicode content when creating file
config: Support UTF-8 in default config

Fixes issue reported at https://discuss.mopidy.com/t/428.

Mopidy-HTTP-Kuechenradio includes a non-ASCII UTF-8 character in its
default config. If Mopidy didn't already have a config file, it crashed
when trying to create the initial config file based on the default
config of all available extensions.