Support Handlebars 4 (or backport patches)

[wvengen]

Development only happens the the latest Handlebars version, which also means that security vulnerabilities are addressed only there. To stay safe, rendr-handlebars needs to either support Handlebars 4, or keep its own branch of Handlebars 2 with security patches. I suggest the former.

Unfortunately, it's not merely a question of upgrading Handlebars, in my last attempts templates could not be found anymore in rendr's shared/base/view.js.

[wvengen]

To be more precise:
https://nodesecurity.io/advisories/handlebars_quoteless-attributes-in-templates-can-lead-to-xss