Some important changes to Image sizes...

1. If you are on a Content page (e.g. the article page) then use Main Picture

2. If you are on an Index page (e.g. tag page or front) then use Trail Picture

3. Profiles now understand how to find the best image based on their width. We were for example getting the largest image and then knocking it down to 220px. We now will use the closest crop equal to or larger than 220px. This should considerably ease load on the image server.
Merge branch 'master' of github.com:guardian/frontend into image-sizes

Conflicts:
	common/test/views/support/ImgSrcTest.scala
Push the image container option into the html fragment
Leave the image server safe state off for now
Relocate element mapping to reusable trait.

Prefer using fixed relations instead of overrides, to keep image logic simple.
Merge conflicts
Largest crops should be used by Gallery only
Fix tests for gallery