devrooms-cfp: update automotive devroom deadline
devrooms-cfp: rename 'legal issues' to 'legal and policy issues'
schedule_helpers: fix title attribute of event times

Because there is no item[:conference_day], ltt() always produced a iso8601
string with today's date.  Not only was this obviously wrong, it also caused
just about every page to need regenerating daily.

While here: simplify the function to reflect its actual use.