Refactor last_fm configuration

Move all configuration outside of the LastFm module, so that it's not
polluted with references to the parent music_library application.

config/dev.exs

@@ -62,6 +62,9 @@ config :music_library, MusicLibraryWeb.Endpoint,
 # Enable dev routes for dashboard and mailbox
 config :music_library, dev_routes: true
 
+config :music_library, LastFm,
+  refresh_interval: System.convert_time_unit(500, :second, :millisecond)
+
 # Do not include metadata nor timestamps in development logs
 config :logger, :console, format: "[$level] $message\n"