feat(admin): edit category content in the admin form behind the HTML gate

- add a content textarea (size huge) after description, with en and pl phrases
- add content to Category.Fillable
- export classes.CheckHTML and run it from FormBeforeCreate and FormBeforeUpdate:
  unsafe category content answers 422 on content, blank content is stored as NULL

classes/format_html.go

@@ -37,6 +37,13 @@ func FormatHTML(src string) (string, error) {
 	return html, nil
 }
 
+// CheckHTML applies the FormatHTML safety gate to HTML that is stored as is,
+// such as category content. It refuses raw script, iframe, object and embed
+// tags, event handler attributes and javascript, vbscript or data URL schemes.
+func CheckHTML(html string) error {
+	return rejectUnsafe(html)
+}
+
 func rejectUnsafe(html string) error {
 	if markdownUnsafeTag.MatchString(html) {
 		return fmt.Errorf("journal: rendered HTML contains raw unsafe tags")