Resolve "event registration information grouping"

Use cases for event registration information on events page

• There are new use cases for the event registration information given in the front matter of a event page:
  • Registration link is given but no registration period.
  • Waiting list link is given but no registration period.
• Fixed a bug in a conditional statement that prevented the event registration badges to be displayed in the event data header.
• TODO: Refine appearance of event registration badges now that they are displayed.

Closes #180 (closed)