Integrate MusicBrainz events into ListenBrainz

MusicBrainz contains data about upcoming and past events. This data is not surfaced, nor available to ListenBrainz users in any way. This data should be utilized in ListenBrainz to provide it's users with an easy way to discover events. This will be achieved by copying over, and caching the event data from MusicBrainz in a suitable format, and creating endpoints, as well as adequate frontend to display this information. The user should be able to discover events, gather event data from ListenBrainz itself, and find events relevant to them based on their preferred artists.