Templates import/export follow-up - create build that imports old templates

[CarlosNZ]

To do after #1025

Modify front-end to send template imports to old endpoint so we have a mechanism to convert from old to new templates. (Modifications currently in branch 1025B-template-export-pt2, so just copy from here into build branch)

Then make a build of this version, very clearly labelled, before reverting changes.