Add dynamic CTA buttons to event hompages

[mattstratton]

For example, "Register" should only show up if registration is enabled, submit, etc, and follow only if a twitter account is set.

[mattstratton]

idea - make a new element called registration_open and if it's set to true, that enables the register menu with the default link
if the optional link registration_link is enabled, then the menu uses that (and the CTA button too)

[mattstratton]

This depends on the following elements in the datafile:

event_twitter: 'devopsdays (without the @ sign)
registration_open: 'false'
registration_link: "" (this is an optional link if they want off-site registration)
cfp_open: "false"
cfp_link: ""

Everything works if these elements don't exist; the CTA buttons just don't display.

Connected to #167