Support special characters in key/value input fields

Values containing special characters like "{" are not supported in key/value input fields.

As an example, the following operation is possible in the CLI, but the counterpart in the web console fails:

/server-group=main-server-group/jvm=default:write-attribute(name=environment-variables, value={"dummyVariable" => "<span class='hx_errorMsg'>{0}</span>"}