fix registration_closed corner case

[bridgetkromhout]

@mattstratton says it looks like "registration_closed" can take either "true" or "" but not "false". See 7e3b211 for repro. This led to two "Register" buttons.

[mattstratton]

This is happening because the logic with registration_closed is wonky.

My proposal is to remove the field altogether and have the display of the "Register" button be totally date-driven.

It's tricky since registration_date_end is totally optional; although we should probably make it required (if registration_date_start is set, that is) and suggest that you set it for the date of your event?

Or we can still do some quick checks on if its set at all I guess.

[mattstratton]

Ah, the end date is NOT optional:

{{- if $e.registration_date_start -}}
          {{- if and (ge now (time $e.registration_date_start)) (le now (time $e.registration_date_end)) -}}