UX: remove "at" word from relative dates in local dates

Previously we would render "Today at 2:42 PM"
Now we render "Today 2:42 PM"

This also introduces new strings so it can be properly translated

Finally it introduces tests so we can make sure this keeps working

plugins/discourse-local-dates/test/javascripts/acceptance/local-dates-test.js.es6

@@ -6,9 +6,36 @@ acceptance("Local Dates", {
   settings: { discourse_local_dates_enabled: true },
   beforeEach() {
     clearPopupMenuOptionsCallback();
+  },
+  afterEach() {
+    sinon.restore();
   }
 });
 
+test("at removal", assert => {
+  let now = moment("2018-06-20").valueOf();
+  let timezone = moment.tz.guess();
+
+  sinon.useFakeTimers(now);
+
+  let html = `<span data-timezones="${timezone}" data-timezone="${timezone}" class="discourse-local-date past cooked-date" data-date="DATE" data-format="L LTS" data-time="14:42:26"></span>`;
+
+  let yesterday = $(html.replace("DATE", "2018-06-19"));
+  yesterday.applyLocalDates();
+
+  assert.equal(yesterday.text(), "Yesterday 2:42 PM");
+
+  let today = $(html.replace("DATE", "2018-06-20"));
+  today.applyLocalDates();
+
+  assert.equal(today.text(), "Today 2:42 PM");
+
+  let tomorrow = $(html.replace("DATE", "2018-06-21"));
+  tomorrow.applyLocalDates();
+
+  assert.equal(tomorrow.text(), "Tomorrow 2:42 PM");
+});
+
 test("local dates bbcode", async assert => {
   await visit("/");
   await click("#create-topic");