Refactored Topic#limit_topics_per_day to improve Code Climate Complexity

Extracted/Refactored 
1) Topic#apply_per_day_rate_limit_for, method as generic RateLimiter 
2) Topic#limit_first_day_topics_per_day as a separate private method,
3) Added User#added_a_day_ago?
4) Fixed private methods indentation.

Would appreciate feedback   :smile: