_layouts/events.html use Nav include and front matter conditional logic

[mvogelgesang]

• [x] Update page to utilize nav.html as an include

Put in conditional logic for all front matter variables found in events/template.html so that Jekyll still renders the page even if a front matter variable is missing.

• [x] title
• [x] datetimestart
• [x] datetimeend
• [x] location
• [x] contactemail
• [x] tags