FEATURE: autocomplete usernames early in topic based on participation

Following this change when a user hits `@` and is replying to a topic they
will see usernames of people who were last seen and participated in the topic

This is somewhat experimental, we may tweak this, or make it optional.

Also, a regression in a423a938 where hitting TAB would eat a post you were writing:

Eg this would eat a post:

``` text
@hello, testing 123 <tab>
```

test/javascripts/lib/user-search-test.js.es6

@@ -1,5 +1,4 @@
 import userSearch from "discourse/lib/user-search";
-import { CANCELLED_STATUS } from "discourse/lib/autocomplete";
 
 QUnit.module("lib:user-search", {
   beforeEach() {
@@ -73,7 +72,29 @@ QUnit.test("it strips @ from the beginning", async assert => {
   assert.equal(results[results.length - 1]["name"], "team");
 });
 
-QUnit.test("it does not search for invalid usernames", async assert => {
-  let results = await userSearch({ term: "foo, " });
-  assert.equal(results, CANCELLED_STATUS);
+QUnit.test("it skips a search depending on punctuations", async assert => {
+  let skippedTerms = [
+    "@sam  s", // double space is not allowed
+    "@sam;",
+    "@sam,",
+    "@sam:"
+  ];
+
+  skippedTerms.forEach(async term => {
+    let results = await userSearch({ term });
+    assert.equal(results.length, 0);
+  });
+
+  let allowedTerms = [
+    "@sam sam", // double space is not allowed
+    "@sam.sam",
+    "@"
+  ];
+
+  let topicId = 100;
+
+  allowedTerms.forEach(async term => {
+    let results = await userSearch({ term, topicId });
+    assert.equal(results.length, 6);
+  });
 });