Add a header identifying the backend to all responses.

this can now be logged in Fastly for debugging purposes.
Reverting ellipsis fix.
Changing touchable zone expansion just to mobile.
Merge pull request #6836 from guardian/backend-header

Add a header identifying the backend to all responses.
Revert "Changing touchable zone expansion just to mobile."

This reverts commit 563ed86509bbcfd12bcacd7760e2e6f1e38933da.
only kill epllisis
Merge pull request #6847 from guardian/navigation-fix

Stop mobile nav wiggling about
Do not poll for updates if there will be no updates
Merge pull request #6849 from guardian/do-not-call-front-index-if-not-necessary

Do not poll for updates if there will be no updates
Merge branch 'master' into fix-deduping-issue
Add test for de-duplication behaviour