FIX: Fix local-dates in non-post contexts, and in long topics (#14565)

- Stop looking up the topic title from the DOM. On long topics, the topic title may not be present. Instead, we can store the topic title in a data-title attribute during decorateCookedElement, and then access it later. This approach would also allow us to add customize titles per-local-date in future. If there is no topic title available (e.g. when local dates are used elsewhere in the UI), we use the site name to build a sensible default

- Don't require a postId for creating calendar events. We don't have postIds in non-post contexts. At the moment, the 'download ICS' function will fail without a valid postId, so that will need to be fixed in a future commit.

plugins/discourse-local-dates/config/locales/client.en.yml

@@ -34,3 +34,4 @@ en:
             every_three_months: "Every three months"
             every_six_months: "Every six months"
             every_year: "Every year"
+      default_title: "%{site_name} Event"