Update to handlebars v5

[dtolnay]

Release notes: https://github.com/sunng87/handlebars-rust/releases/tag/v5.0.0

Currently this is blocked on handlebars-fluent and rocket_dyn_templates.

• Update to support handlebars v5 Manishearth/handlebars-fluent#13
• Publish handlebars v5 support Manishearth/handlebars-fluent#14
• Update handlebars to v5 rwf2/Rocket#2700
• New release of rocket_dyn_templates rwf2/Rocket#2793