IssueLabelApp: Rewrite javadoc in English

When a new label is added, encode the label's fields: {@link IssueLabel#id}, {@link IssueLabel#name}, {@link IssueLabel#color}, and {@link IssueLabel#category} into {@code application/json}, and include them in the body of the 201 Created response. But if the client cannot accept {@code application/json}, return the 201 Created with no response body.