signin: Add required property for country (CA)

During the Certificate Authority creation, the admin must choose a country.
This is required and locked, but the property was missing, so the "*" was not displayed.
However, it was already not possible to create a CA without a country; this change is purely visual.

Change-Id: I7a852ccb9d6f13a1552d4d17cab1a8bc1696a463

jams-react-client/src/components/CountrySelect/CountrySelect.tsx

@@ -71,6 +71,7 @@ export default function CountrySelect({
       renderInput={(params) => (
         <TextField
           {...params}
+          required
           label={i18next.t("choose_a_country", "Choose a country") as string}
           name="country"
           variant="outlined"