Use master image for DCR TrailType

[marjisound]

Currently, when converting a Frontend trail type to a DCR trail type, the image
field is used for the image asset. This field contains an i.guim.co.uk URL, which is then converted to a media.guim.co.uk URL in the getSourceImageUrl
function.

We now have a masterImage field in the Frontend trail type that already provides the media.guim.co.uk URL. The plan is to update decideTrail to use masterImage instead of image. When doing this, we’ll also need to ensure that getSourceImageUrl is no longer called, since the conversion will no longer be necessary.

[SiAdcock]

Are we still going with this approach @marjisound, given we have closed #14809?