Flash message no longer says 'true'

This fixes #215 so that a `true` flash message is no longer displayed.
Redirect inbound port 80 to HTTPS

This assumes the ELB is terminating HTTPS and forwarding to port 443 as plain HTTP
Fix redirect; add HSTS
Fix mixed-content warnings over HTTPS

Added placeholder PNGs
HSTS tweak from @konklone
Merge pull request #269 from 18F/hotfix/remove-true-flash-message

Flash message no longer says 'true'
Merge pull request #280 from 18F/feature/redirect-to-https

Feature/redirect to https
Merge branch 'devel' into feature/destroy-account