Refactored Topic#limit_topics_per_day to reduce code climate complexity

Extracted  1) #apply_per_day_rate_limit_for, method as generic
RateLimiter , 2) #limit_first_day_topics_per_day as a separate method,
3) Added User#added_a_day_ago?, 4) Fixed private methods indentation.
Merge pull request #1531 from railsaholic/topics_model_refactoring

Refactored Topic#limit_topics_per_day to improve Code Climate Complexity