Important Check hints for single-location areas

[fenhl]

To reduce redundancy in hints, an Important Check hint for an area with only 1 shuffled location should mark that location as already hinted.

Conversely, an Important Check hint shouldn't generate for an area with all shuffled locations already hinted.

I think this is a decent compromise between the current behavior, where these hints never prevent each other from generating, and the option of having all Important Check hints mark their areas as already hinted.

See today's discussion in #dev-public-talk for more context.