Improvements to hint administration

- Renamed level_source_hints/ to review_hints.
- Added pagination when reviewing hints.
- Added filtering by status.
- Added new status value for new hints (currently null until set by a reviewer).
- Dropped useless columns from display.
- Used source field so only crowdsourced hints are accessible through the GUI.
Ruby symbols are generally a better replacement for these kinds of constants. They're great because you don't need to pre-define them, at runtime they're as fast as integer compares, but they can be displayed as strings. So, instead of:

STATUS_SELECTED = 'selected'
USER_VISIBLE_NAMES = { STATUS_SELECTED => 'approved' }

I would use :approved wherever you were using STATUS_SELECTED
and
I would use variable.to_s wherever you're using USER_VISIBLE_NAMES[variable]