Hide MyMLH button if MLH_KEY env var is not set

[sman591]

Wrap these in a check for ENV["MLH_KEY"].present?

• https://github.com/codeRIT/hackathon_manager/blob/master/app/views/devise/sessions/_form.html.haml#L2
• https://github.com/codeRIT/hackathon_manager/blob/master/app/views/devise/registrations/_form.html.haml#L2

This way, MyMLH support is optional