Migrate to fastly IO CDNs

[Bradpullen]

This change migrates use from an imgix subdomain to a uswitch or money subdomain
img.[brand].com uses Fastly IO to perform image optimisation and caching.

💀 Be warned! The query parameters are different between imgix & Fastly IO.
We have tried our best to map the the values between each so you don't need to update anything, but you should check each image loads as expected. If not, you may need to modify the query params or the image itself.

In particular, look out for

• invert=true
• fit=clamp
• trim=color

Documentation on the mapping can be found here

Created by Sourcegraph batch change Bradpullen/s3-uswitch-assets-to-fastly-io.