MDL-61028 core_search: Allow filtering search by user (front-end)

Creates a new 'Users' field in the search filters form. This field
requires new JavaScript and, to implement this, a new AJAX-callable
web service to search for users by name, with detailed restrictions
based on the current user's access to view profiles.

search/templates/form-user-selector-suggestion.mustache

@@ -0,0 +1,41 @@
+{{!
+    This file is part of Moodle - http://moodle.org/
+
+    Moodle is free software: you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    Moodle is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
+}}
+{{!
+    @template core_search/form-user-selector-suggestion
+
+    Moodle template for the list of valid options in an autocomplate form element.
+
+    Classes required for JS:
+    * none
+
+    Data attributes required for JS:
+    * none
+
+    Context variables required for this template:
+    * profileimageurlsmall Url to a small profile image.
+    * fullname Users full name
+
+    Example context (json):
+    { "id": "1",
+      "fullname": "Kermit Thefrog",
+      "profileimageurlsmall": "https://example.org/img"
+    }
+}}
+<span>
+    <img height="12" src="{{profileimageurlsmall}}" alt="" role="presentation">
+    <span>{{fullname}}</span>
+</span>