Performer custom fields (#5487)

* Backend changes
* Show custom field values
* Add custom fields table input
* Add custom field filtering
* Add unit tests
* Include custom fields in import/export
* Anonymise performer custom fields
* Move json.Number handler functions to api
* Handle json.Number conversion in api

internal/api/json.go

@@ -0,0 +1,36 @@
+package api
+
+import (
+	"encoding/json"
+	"strings"
+)
+
+// JSONNumberToNumber converts a JSON number to either a float64 or int64.
+func jsonNumberToNumber(n json.Number) interface{} {
+	if strings.Contains(string(n), ".") {
+		f, _ := n.Float64()
+		return f
+	}
+	ret, _ := n.Int64()
+	return ret
+}
+
+// ConvertMapJSONNumbers converts all JSON numbers in a map to either float64 or int64.
+func convertMapJSONNumbers(m map[string]interface{}) (ret map[string]interface{}) {
+	if m == nil {
+		return nil
+	}
+
+	ret = make(map[string]interface{})
+	for k, v := range m {
+		if n, ok := v.(json.Number); ok {
+			ret[k] = jsonNumberToNumber(n)
+		} else if mm, ok := v.(map[string]interface{}); ok {
+			ret[k] = convertMapJSONNumbers(mm)
+		} else {
+			ret[k] = v
+		}
+	}
+
+	return ret
+}