Fix add_class_to_el templatetag to work with lists for classes

aleksis/core/templatetags/html_helpers.py

@@ -15,6 +15,6 @@ def add_class_to_el(value: str, arg: str) -> str:
     soup = BeautifulSoup(value, "html.parser")
 
     for el in soup.find_all(el):
-        el["class"] = el.get("class", "") + f" {cls}"
+        el["class"] = el.get("class", []) + [cls]
 
     return str(soup)