MDL-63103 filebrowser: Speed up finding activities with files

Before this change the query examined at every course_module record in
the context table.

With this change we use the course_modules table to more efficiently
find all the activities in the course, which means that many less
records in the context table need to be examined.

lib/filebrowser/file_info_context_course.php

@@ -528,16 +528,15 @@ class file_info_context_course extends file_info {
         $params1 = ['contextid' => $this->context->id,
             'emptyfilename' => '.',
             'contextlevel' => CONTEXT_MODULE,
-            'depth' => $this->context->depth + 1,
-            'pathmask' => $this->context->path . '/%'];
+            'course' => $this->course->id];
         $sql1 = "SELECT ctx.id AS contextid, f.component, f.filearea, f.itemid, ctx.instanceid AS cmid, " .
                 context_helper::get_preload_record_columns_sql('ctx') . "
             FROM {files} f
             INNER JOIN {context} ctx ON ctx.id = f.contextid
+            INNER JOIN {course_modules} cm ON cm.id = ctx.instanceid
             WHERE f.filename <> :emptyfilename
-              AND ctx.contextlevel = :contextlevel
-              AND ctx.depth = :depth
-              AND " . $DB->sql_like('ctx.path', ':pathmask') . " ";
+              AND cm.course = :course
+              AND ctx.contextlevel = :contextlevel";
         $sql3 = ' GROUP BY ctx.id, f.component, f.filearea, f.itemid, ctx.instanceid,
               ctx.path, ctx.depth, ctx.contextlevel
             ORDER BY ctx.id, f.component, f.filearea, f.itemid';