[CR] Sleek Scales morale-effect buff

This makes morale bonuses noticeably easier to get as a Fish; since Sleek Scales is Fish-exclusive, I don't think it should be a major balance issue.

This and #4745 are in response to
http://smf.cataclysmdda.com/index.php?topic=3982.msg65591#msg65591 ; I'm not sure I've got a good handle on how i2amroy's initial code works, so I'm hesitant to just start breaking it.  (In particular, I'd thought of making mutcat_fish automatically water_friendly.)

I believe, and am guessing others believe, that water-unfriendly clothing getting Drenched would make it clammy, cumbersome, and Unfun, which is why I took this approach.

As the tag implies, comments requested.