Mixed http/https content warnings

[h-lame]

Netlify have a new feature where the build can tell you about mixed content in your generated pages. Turns out, we've got a bunch. It's mostly images hosted elsewhere that are http only - we might be able to just turn them into https instead, or try the s3-ify script to move them to S3 where we should be able to load them via https instead.

Here's the list generated by this build:

2:44:06 PM: Mixed content detected in: /blog/corporate-to-unboxed-a-project-manager-s-journey-to-scrum-master-part-1/index.html
2:44:06 PM: --> insecure img urls:
2:44:06 PM:   - http://bit.ly/1Q9FtWl
2:44:06 PM:   - http://bit.ly/1Igpq1O
2:44:06 PM:   - http://bit.ly/1Fwcb9U
2:44:06 PM:   - http://bit.ly/1KcgIDX
2:44:06 PM: Mixed content detected in: /blog/corporate-to-unboxed-part-2-advanced-scrum-master-training/index.html
2:44:06 PM: --> insecure img urls:
2:44:06 PM:   - http://bit.ly/1eGZ7sb
2:44:06 PM:   - http://bit.ly/1RvFDmk
2:44:06 PM:   - http://bit.ly/1BLWG1J
2:44:06 PM:   - http://bit.ly/1N9O2uk
2:44:06 PM:   - http://bit.ly/1Gyo6Uc
2:44:06 PM: Mixed content detected in: /blog/culture/page-5/index.html
2:44:06 PM: --> insecure img urls:
2:44:06 PM:   - http://unboxed.co/assets/images/blog/Ruby_Team.png
2:44:06 PM: Mixed content detected in: /blog/culture/page-6/index.html
2:44:06 PM: --> insecure img urls:
2:44:06 PM:   - http://bit.ly/1iCddwP
2:44:07 PM: Mixed content detected in: /blog/one-month-and-eight-arms-starting-at-unboxed-consulting-and-launching-tentacles/index.html
2:44:07 PM: --> insecure img urls:
2:44:07 PM:   - http://f.cl.ly/items/1V2j3z163i31370k380M/tentacles-wall.jpg
2:44:07 PM: Mixed content detected in: /blog/page-6/index.html
2:44:07 PM: --> insecure img urls:
2:44:07 PM:   - http://unboxed.co/assets/images/blog/Ruby_Team.png
2:44:07 PM: Mixed content detected in: /blog/page-7/index.html
2:44:07 PM: --> insecure img urls:
2:44:07 PM:   - http://i963.photobucket.com/albums/ae115/Ubxd/martyn-speaking-colombia_zpscahcr9us.jpg
2:44:07 PM: Mixed content detected in: /blog/page-8/index.html
2:44:07 PM: --> insecure img urls:
2:44:07 PM:   - http://bit.ly/1iCddwP
2:44:07 PM: Mixed content detected in: /blog/summer-at-unboxed-round-two/index.html
2:44:07 PM: --> insecure img urls:
2:44:07 PM:   - http://bit.ly/1KxOr7U
2:44:07 PM:   - http://bit.ly/1PkrPem

[h-lame]

We'll also want to change the image_url helper method to use https as well.

[h-lame]

Addressed some of this in #501, and #503. Addressing some more in #519.