Revert "Change link to Transactions Explorer"

This reverts commit 7a6440a4974da1bd86aefdbe90c5bb1fa93acc48.
Remove annotations_from_backdrop feature toggle

It's in production, has been for a while and works.
Add new setting for an external Backdrop hostname

Bug fix. This app requests data from Backdrop in two places: internally,
and via AJAX in the browser. These URLs are routed differently in different
environments (e.g. www.gov.uk is not routed internally in production).
Merge pull request #34 from alphagov/fix-insidegov-annotations-config

Fix government annotations on all browsers [#55480996]