UX: Various improvements to welcome topic CTA (#21010)

- Update welcome topic copy
- Edit the welcome topic automatically when the title or description changes
- Remove “Create your Welcome Topic” banner/CTA
- Add "edit welcome topic" user tip

spec/requests/api/schemas/json/site_response.json

@@ -241,6 +241,9 @@
         },
         "suggested_topics": {
           "type": "integer"
+        },
+        "welcome_topic": {
+          "type": "integer"
         }
       },
       "required": [
@@ -752,9 +755,6 @@
           ]
         }
     },
-    "show_welcome_topic_banner": {
-      "type": "boolean"
-    },
     "archetypes": {
       "type": "array",
       "items":