feat(Handlebars): formatNumber and group helpers

[Vitor-Avila]

SUMMARY

This PR adds two new helpers to the Handlebars chart:

• formatNumber: allows using number.toLocaleString(locale) to format numbers as needed.
• group (via the handlebars-group-by module): Allows grouping data.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

Example using both helpers:

TESTING INSTRUCTIONS

1. Create a Handlebars chart using the cleaned_sales_data dataset.
2. Use year and deal_size as dimensions and create any metric with the test_metric label.
3. Use below Handlebars template:

{{#group data by="year"}}
<h3>Year: {{ value }}</h3>
<ul>
{{#each items}}
	<li>{{ deal_size }} deals: {{ formatNumber test_metric "de-DE"}}</li>
{{/each}}
</ul>
{{/group}}

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[kgabryje]

LGTM

[jediwarpraptor]

This is phenomenal @Vitor-Avila - HUGE! 🎉 🚀 💜

[rusackas]

Love it!
• Could add a test, but it would be a pretty low-likelihood-to-fail test since it would just test that JS does JS stuff.
• We're low on docs for this thing, but this is a reminder that we could really use some, and I'd say we should update them if we had them at all.

[Vitor-Avila]

@rusackas 100% agreed. I just took a look at https://superset.apache.org/docs/using-superset/exploring-data, and we don't have a section for Handlebars just yet. I'll see if I can work on a follow up PR covering docs (and perhaps testing).