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/test/javascripts/acceptance/download-calendar-test.js.es6

@@ -57,7 +57,7 @@ acceptance(
       assert.ok(!exists(document.querySelector("#discourse-modal-title")));
       assert.ok(
         window.open.calledWith(
-          "https://www.google.com/calendar/event?action=TEMPLATE&text=Local%20dates%20&dates=20210930T110000Z/20210930T120000Z",
+          "https://www.google.com/calendar/event?action=TEMPLATE&text=Local%20dates&dates=20210930T110000Z/20210930T120000Z",
           "_blank",
           "noopener",
           "noreferrer"