FEATURE: Decorate topic-level bookmark button with reminder time (#9426)

* Show the correct bookmark with clock icon when topic-level bookmark reminder time is set and show the time of the reminder in the title on hover.
* Add a new bookmark lib and reminder time formatting function to show time with today/tomorrow shorthand for readability. E.g. tomorrow at 8:00am instead of Apr 16 2020 at 8:00am. This only applies to today + tomorrow, future dates are still treated the same.

test/javascripts/lib/bookmark-test.js

@@ -0,0 +1,51 @@
+import { formattedReminderTime } from "discourse/lib/bookmark";
+
+QUnit.module("lib:bookmark", {
+  beforeEach() {
+    // set the current now time for all tests
+    let now = moment.tz("2020-04-11 08:00:00", "Australia/Brisbane");
+    sandbox.useFakeTimers(now.valueOf());
+  }
+});
+
+QUnit.test(
+  "formattedReminderTime works when the reminder time is tomorrow",
+  assert => {
+    let reminderAt = "2020-04-12 09:45:00";
+    let reminderAtDate = moment
+      .tz(reminderAt, "Australia/Brisbane")
+      .format("H:mm a");
+    assert.equal(
+      formattedReminderTime(reminderAt, "Australia/Brisbane"),
+      "tomorrow at " + reminderAtDate
+    );
+  }
+);
+
+QUnit.test(
+  "formattedReminderTime works when the reminder time is today",
+  assert => {
+    let reminderAt = "2020-04-11 09:45:00";
+    let reminderAtDate = moment
+      .tz(reminderAt, "Australia/Brisbane")
+      .format("H:mm a");
+    assert.equal(
+      formattedReminderTime(reminderAt, "Australia/Brisbane"),
+      "today at " + reminderAtDate
+    );
+  }
+);
+
+QUnit.test(
+  "formattedReminderTime works when the reminder time is in the future",
+  assert => {
+    let reminderAt = "2020-04-15 09:45:00";
+    let reminderAtDate = moment
+      .tz(reminderAt, "Australia/Brisbane")
+      .format("H:mm a");
+    assert.equal(
+      formattedReminderTime(reminderAt, "Australia/Brisbane"),
+      "at Apr 15, 2020 " + reminderAtDate
+    );
+  }
+);