New metrics: Muted / ignored users

[ghost]

When issue #31 is complete, we can add metrics related to muted / ignored users, which are the opposite of followed user metrics:

• post_up_voted_num_muted: Number of muted voters who up voted on post
• post_down_voted_num_muted: Number of muted voters who down voted on post
• post_up_voted_any_muted: At least one muted voter up voted on post
• post_down_voted_any_muted: At least one muted voter down voted on post
• author_is_muted: Author is muted