FEATURE: navigate to first post and auto bump category settings

### navigate_to_first_post_after_read setting for categories

When enabled on categories logged on users will return to OP after
reading the entire category. (useful for documentation categories)

### num_auto_bump_daily

Set a number of topics that will automatically bump daily on a category.

- Every 15 minutes we will check if any category has this setting
- Categories with the setting are shuffled
- We exclude pinned, closed, category description and archived topics
- Maximum of 1 topic for the list of categories is bumped till limit reached per category
- We always try to bump oldest first
- Limit is elastic using a RateLimiter that ensures that we only bump N per day

Also some minor organisation on category settings

Froze strings on category.rb

test/javascripts/models/topic-test.js.es6

@@ -32,6 +32,23 @@ QUnit.test("visited", assert => {
   );
 });
 
+QUnit.test("lastUnreadUrl", assert => {
+  const category = Em.Object.create({
+    navigate_to_first_post_after_read: true
+  });
+
+  const topic = Topic.create({
+    id: 101,
+    highest_post_number: 10,
+    last_read_post_number: 10,
+    slug: "hello"
+  });
+
+  topic.set("category", category);
+
+  assert.equal(topic.get("lastUnreadUrl"), "/t/hello/101/1");
+});
+
 QUnit.test("has details", assert => {
   const topic = Topic.create({ id: 1234 });
   const topicDetails = topic.get("details");