Will not accept a '%' in metadata text

[pikesley]

How to reproduce:

• POST to e.g. https://sport-england-bothan.herokuapp.com/metrics/operating-cost/metadata with JSON like

{
    "description": {
        "en": {
            "Full Year Forecast - % of expenditure"
        }
    }
}

Get a 500. In the logs:

Unexpected error while processing request: invalid %-encoding ({"title":{"en":"Operating Cost Target"},"description":{"en":"Full Year Forecast - % of expenditure"},"type":"","datatype":""})