Fixes a number of things in the simple sleep entry.

Including:
*make a better form for sleeps, that does the validation and conversion
to datetimes correctly, including timezone checking and overlap checking
(no need for an override as that's actually impossible now that we use
timezones)
*continues to work with 12/24 hour time
*URL is now /sleep/simple/ because it's nicer
*Now linked from /mysleep/

In general, anything that can use sleep.forms.SleepForm should, since it
will handle validation and timezones correctly.  At some point we should
change the grid interface to use that.