Improve translation handling in JavaScript and TypeScript

[hansegucker]

Closes #2027

[niklasmohrin]

For Typescript and modules:

1. It seems that using a global function from a module just works, so we can keep using the import as it currently is
2. To inform Typescript about the existence of these functions, a line like the following can be added and imported in all files using it:

export declare function gettext(s: string): string;

I would suggest that these declarations live either in utils.ts or in a separate file like translation.ts, slightly preferring the latter.

[hansegucker]

@niklasmohrin Ready for review

[niklasmohrin]

Very cool!

[richardebeling]

Looking good, thanks :)

[hansegucker]

@richardebeling

[niklasmohrin]

Thanks :)

[richardebeling]

Thanks :)