Feedback collect

1. Added a rake task :frequent_level_sources[freq_cutoff] that adds the frequently appeared error programs into table 20140205220657_create_frequent_unsuccessful_level_sources.rb, and marks the ones that do not have at least three hints as active ones.
2. Added a page http://localhost:3000/frequent_unsuccessful_level_sources where teachers can add hints for all programs for a specific level. Each time a hint is added, that level is marked as inactive.

All pages are currently only accessible by admin users. Will change to teachers once we have the final version.

TODO:
This is for functionality demonstration. Finer tune will be needed once the general framework is approved (e.g., a link to show all the hints. Add_pop_hint and add_hint have redundant code, will merge or remove one based on whether we'd like to keep the add_hint page.).