file_icons: Use a separate icon key for HTML files (#24323)

This PR updates the file icon mappings such that HTML (`.html` and
`.htm`) files map to the `html` key.

This allows for the HTML file icons to be replaced in icon themes.

Release Notes:

- Icon themes: Added the ability to change the file icon for HTML
(`.html`, `.htm`) files.

assets/icons/file_icons/file_types.json

@@ -86,8 +86,8 @@
     "hpp": "cpp",
     "hrl": "erlang",
     "hs": "haskell",
-    "htm": "template",
-    "html": "template",
+    "htm": "html",
+    "html": "html",
     "hxx": "cpp",
     "ib": "storage",
     "ico": "image",