Create a placeholder image for mentors who don't supply photos

[keithamoss]

Do we need it?

For the mentors bar.

[alansfyeung]

This got done over the last couple of weekends. If there's no image defined in frontmatter then the template uses the default image.

The image for "anonymous sponsor" is:
https://s3-ap-southeast-2.amazonaws.com/2016govhackmentors/secret-squirrel.jpg

[keithamoss]

That works well for sponsors, but I had in mind a more "silhouette of person" placeholder for mentors. Reopening and assigning to myself as a lower priority.