page_params: Add community_topic_editing_limit_seconds.

This was previously hardcoded with agreement between the Zulip backend
and frontend as 86400 seconds (1 day).  Now, it's still hardcoded in
the backend, but arranged in a way where we could add a setting
without any changes to the mobile and terminal apps to update logic.

Fixes #15278.

static/js/message_edit.js

@@ -47,9 +47,9 @@ function is_topic_editable(message, edit_limit_seconds_buffer) {
     }
 
     // If you're using community topic editing, there's a deadline.
-    // TODO: Change hardcoded value (24 hrs) to be realm setting.  Currently, it is
-    // DEFAULT_COMMUNITY_TOPIC_EDITING_LIMIT_SECONDS
-    return 86400 + edit_limit_seconds_buffer + now.diffSeconds(message.timestamp * 1000) > 0;
+    return page_params.realm_community_topic_editing_limit_seconds
+        + edit_limit_seconds_buffer
+        + now.diffSeconds(message.timestamp * 1000) > 0;
 }
 
 function get_editability(message, edit_limit_seconds_buffer) {