Add 'tax_table' format to tax block

We now offer a "tax_table" format to display a table of bands and rates
using a single embed code, e.g.

```
{{embed:content_block_tax:income-tax#tax_table}}
```

Given a tax object for income tax as per https://www.gov.uk/income-tax-rates
this will render a table of 12 values like:

```

| Band               | Taxable income      | Tax rate |
| ------------------ | ------------------- | -------- |
| Personal Allowance | Up to £12,570       | 0%       |
| Basic rate         | £12,571 to £50,270  | 20%      |
| Higher rate        | £50,271 to £125,140 | 40%      |
| Additional rate    | over £125,140       | 45%      |

```

See [Content Block Tools PR 155][] for the tax table format

[Content Block Tools PR 155]:
alphagov/govuk_content_block_tools#155

Author: edavey

app/models/schema/definitions/tax.json

@@ -2,6 +2,7 @@
   "type": "object",
   "required": ["tax_type"],
   "additionalProperties": false,
+  "x-formats": ["tax_table"],
   "x-field-order": ["title", "abbreviation", "synonym", "tax_type", "description", "note"],
   "properties": {
     "abbreviation": {