Use Mantle to load front-end handlebars resources

WIP, depends on gerrit Ia978e6ec9 which adds functionality to Mantle name
handlebars templates as ResourceLoader templates, and compile them.
This duplicates similar functionality in flow-handlebars.js, which Flow
uses for progressive enhancement.

Change-Id: Ifd3a95e0140783ffd680d41f881f8f8a808577da
Merge "Use Mantle to load front-end handlebars resources" into frontend-rewrite