Only list hackathons in digests every 3 months if it's more than a month out

[northeastprince]

No description provided.

[garyhtou]

I like this idea! To make sure we're on the same page:

• If hackathon is less than 1 month from now, list it every time
• If hackathon is more than 1 month from now, list it every 3 months

Maybe we should update it to every 1 month

[garyhtou]

Another way to approach this problem is:

# sudo code

def black_list # there's prob a better name
  # hackathons that are more than 1 month away and have been listed within the past 3 months for this recipient
end

applicable_listings.remove(black_list).first(MAX_LISTINGS)

This would make fewer queries to the DB I think

[garyhtou]

Is there anything blocking this PR?