Replace http URLs by relative URLs

[duncanpeacock]

This is a follow on from issue #226 and relates to new security rules built into the latest version of Chrome.

In the Fragalysis Backend, all formatted URLS that have http in them should be changed to relative URLs.
These will likely affect models that that Filefields in them (it's default the serialization behaviour for these) - for example, the ones ending in /media/target/xxxx.zip and /media/target/metadata.csv.

Note that this change should be coordinated with the frontend as some URLs from the backend are modified in transit. We don't want to break these.