Removed route mapping /hint -> /frequent_unsuccessful_level_sources
Various improvements:
- Changed more hint_path to frequent_unsuccessful_level_sources_path.
- Added links to code.org/hints.
- Added links from bottom middle of add_hint.html.haml and add_pop_hint.html.haml.
- Made show_pop_hints.html.haml use common code.
- Removed some unneeded hardcoded text from show_pop_hints.html.haml.
- Put back links removed earlier.
Added comments, fixed broken test.