Async helpers

[groenroos]

The get helper needs to use Promises in order to fetch data from the database, but Handlebars doesn't seem to support async/await helper callbacks out-of-the-box.

Fix it so that the get callback can output async data into the template.

[groenroos]

Without having tested anything, there is handlebars-async (which seems to be unmaintained), and express-hbs (which is more popular, but fundamentally a very different approach).

There is also the possibility of storing the pending Promises centrally, and only executing the rendered() template function once they have all resolved.