Improvements to hint UI

- Moved editable hint table into new partial (_edit_hint_table.html.haml).
- Referenced partial from review_hints (from which it was removed) and show_hints (new).
- Renamed variable @level_source_hints to @hints (because I accidentally got the whole
  database table when I forgot to initialize it).
- Renamed /frequent_unsuccessful_level_sources to /hints.
- Removed infrequently used links and unneeded columns.
- Improved messaging around empty tables and restrictions.