search_suggestion: Show profile pictures in autocomplete suggestions.

Because the typeahead.js list items are currently just text, a user's
full name and avatar should be displayed in `input_pill`. To use
`input_pill`, a separate Handlebars partial view was created to
provide a mandatory container (`<div class="pill-container">`) for
`input_pill` and a flex container (`<div class="search_list_item">`)
for vertically aligning the text.

The description of each suggestion (i.e `description_html`) is
rendered as raw HTML, so every special character (e.g. whitespace)
should be HTML-escaped. This enables highlighting the substring in
each search suggestion that matches the query.

Fixes: #20267

static/js/search.js

@@ -1,5 +1,7 @@
 import $ from "jquery";
 
+import render_search_list_item from "../templates/search_list_item.hbs";
+
 import {Filter} from "./filter";
 import * as message_view_header from "./message_view_header";
 import * as narrow from "./narrow";
@@ -94,7 +96,7 @@ export function initialize() {
         naturalSearch: true,
         highlighter(item) {
             const obj = search_map.get(item);
-            return obj.description_html;
+            return render_search_list_item(obj);
         },
         matcher() {
             return true;