Schedule UI improvements

[pcantrell]

A grab bag:

• Schedule page now works even if sessions don’t have rooms yet. This is in anticipation of getting the schedule out early this year, and assigning rooms only later.
• The schedule — and the whole app! — should always show dates in Central Time, regardless of the user’s device’s time zone.
• Fixed broken CSS/JS includes (probably got hosed during the Rails 5 migration?).
• Mopped up some layout stuff that got messy with incremental fixes over the years.
• Fixed broken URL in http → https admin redirect. (Should we force https site-wide?)

[coveralls]

Coverage increased (+0.4%) to 56.062% when pulling 3372a92 on schedule-ui-improvements into c714c76 on master.

[coveralls]

Coverage increased (+0.3%) to 55.982% when pulling 3372a92 on schedule-ui-improvements into c714c76 on master.

[jcoyne]

It would be helpful to say that we use this in preload_attendance_counts as an optimization.

[jcoyne]

By in large these changes look good. Generally it would be helpful to other maintainers if you would add a bit more method documentation.

[caseyhelbling]

LGTM

[pcantrell]

I added a few light comments. Justin, LMK if there's stuff left that still seems unclear to you.

[jcoyne]

@pcantrell thanks man! 😎