Improvements to hint UI

- Created and moved duplicated code into new partials:
  - _add_hint.html.haml
  - _edit_hint_table.html.haml
  - edit_hint_table_status_button.html.haml
- Renamed variable @level_source_hints to @hints (because I accidentally got the whole
  database table when I forgot to initialize it).
- Removed infrequently used links and unneeded columns.
- Improved messaging around empty tables, restrictions, and hint solicitation.
- Added new view show_hints (which I may later merge with an existing view).
TODO:
- i18n
- Facilitate reviewing hints by enabling restrictions on more views, not just review_hints.

See this diagram, which helped me understand the flow among pages: https://docs.google.com/a/code.org/drawings/d/1yidY-TLCtQODoNyfMgfrSdo1-MspOGrCNkxD1vJybZw/edit?usp=sharing
Might be worth renaming the local variable `idx` to something more descriptive, hard to tell what it means in this context (though the param would be tougher to change). Maybe `popularity_index`?