feat: articles format option with markdown new default

api/pkg/apperr/messages.go

@@ -34,3 +34,4 @@ const UpdateAlreadyRunningErrorMessage = "An update is already running or waitin
 const NothingToUpdateErrorMessage = "There is nothing to update."
 const UpdateRefusedErrorMessage = "This update cannot be downloaded."
 const UpdateRestartMismatchErrorMessage = "The update state changed. Reload the page and try again."
+const ArticleBodyFormatErrorMessage = "The format must be one of: markdown, html, json."