MDL-31989 search: Search API and search engine API

Introducing both APIs in moodle along with:
- search_box widget to add a tiny search box
- admin settings with setup steps helper
- cache for search results
- template for a search result
- php unit stuff

Many thanks to Tomasz Muras, Prateek Sachan and Daniel Neis for their contributions, for starting this development
and for pushing for it to be completed. Also thanks to other contributors: Jonathan Harker and eugeneventer.

lib/phpunit/classes/util.php

@@ -215,6 +215,7 @@ class phpunit_util extends testing_util {
         get_message_processors(false, true, true);
         filter_manager::reset_caches();
         core_filetypes::reset_caches();
+        \core_search\manager::clear_static();
 
         // Reset static unit test options.
         if (class_exists('\availability_date\condition', false)) {