On top this fix, this commit adds support for "My content" and
"Favorites" instance radios (fixes#51), as well as clearly separates instance
radios from user radios.
Radios were a bit unusable when not logged in with an actual authorized
user account, this commit fixes the following elements:
* Anonymous users get a transient session cookie when starting a radio
session that was not stored and forwarded on playback, meaning no
radios would play;
* Anonymous users do not have their own own content. Thus, only the
"Random" radio makes sense in that context. This commit only display
the instance radios that are relevant to your authentication status.
"My content" radios needs the user ID to function properly, this commit
also adds retrieving it from the /api/v1/users/users/me/ endpoint, which
now may be used in the future for other purposes.
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