Support language-specific pluralization

[quincylvania]

Closes #597.

This is essentially a reimplementation of #4964 (shout-out to @1ec5!), but for the current codebase.

We can also use the built-in Intl.PluralRules function instead of an external dependency, since it's now supported in all major browsers. iD will attempt to fallback gracefully on legacy browsers.

[1ec5]

Looks like iD uses Ruby-style YAML, so Transifex should recognize the one and other subkeys as plural rules. 👍

[1ec5]

This might be a good place to apply some number formatting both for grouping characters (really big changesets!) and non-European numerals (for languages like Arabic and Hindi).

[quincylvania]

Agreed, this is why I left strings like {n} result for {search} instead of 1 result for {search}. I think I'll add formatting separately though.

[quincylvania]

Looks like iD uses Ruby-style YAML, so Transifex should recognize the one and other subkeys as plural rules. 👍

@1ec5 Yep, I made the changes with that documentation in mind. I'll debug if Transifex doesn't pick it up for some reason.

[quincylvania]

Looks like Transifex is reacting as expected!

[quincylvania]

@1ec5 Do you know if we need to enable plural forms for strings like this that are based on an amount but don't include the amount?

      too_bendy:
        single: This can't be straightened because it bends too much.
        multiple: These can't be straightened because they bend too much.

[maro-21]

Do you know if we need to enable plural forms for strings like this that are based on an amount but don't include the amount?

      too_bendy:
        single: This can't be straightened because it bends too much.
        multiple: These can't be straightened because they bend too much.

It's not necessary when there is no numeral.