Extend username mention notifications to topics

Currently, username mentions only work in comments. It would be nice to have them work from topics as well (inside the text of text topics, I don't think titles should have mentions in them).

This probably won't be too bad to implement, but will require either changing the consumer to be able to handle both topics and comments, or adding a second topic-specific consumer. The trickier part might be changing the notifications page, since currently it only ever displays comments. There's also only a comment_notifications table and not topic_notifications yet, so overall a reasonable number of changes would be required around this.