System messages

[eevee]

Gotta tell peeps what's happening.

Example events from the old ticket:

• someone rates your art
• someone albums your art
• someone watches you
• someone claims you collaborated on an art

Comments are a bit more complex:

• someone replies to your art
• someone replies to your journal
• someone comments on your userpage
• someone replies to your comment on not-your artwork
• someone replies to your comment on not-your journal
• someone replies to your comment on not-your userpage

Torn on whether this is 0.5 or 0.6, but I guess it's not strictly necessary.

[epii]

Presumably we wouldn't actually disclose who rated your art in the message. Would we?

[eevee]

Probably not. Even if we did, that would just be a landslide of noise that most people wouldn't even look at. I think we may want to notify of new ratings (and faves/albumings, watches, etc.) in aggregate. dA has something like this but it's kinda dumb and awkward and bad.