MDL-55866 mod_data: Remember editor enabled state for templates

Some templates can legitimately contain invalid HTML (e.g. mismatched tags)
because the templates operate in pairs.

In these instances we should not use an editor because the nature of the
editor (content editable) means that the browser automatically corrects all
HTML supplied to it, thus breaking the template entirely.

Therefore we need to disable HTML editors for some templates, and do so in
a way tied to the instance of the activity, rather than to a specific user.

This patch adds a new 'config' field, with matching setters and getters, to
allow such per-instance values to be stored.

mod/data/lib.php

@@ -4045,3 +4045,49 @@ function data_refresh_events($courseid = 0) {
     }
     return true;
 }
+
+/**
+ * Fetch the configuration for this database activity.
+ *
+ * @param   stdClass    $database   The object returned from the database for this instance
+ * @param   string      $key        The name of the key to retrieve. If none is supplied, then all configuration is returned
+ * @param   mixed       $default    The default value to use if no value was found for the specified key
+ * @return  mixed                   The returned value
+ */
+function data_get_config($database, $key = null, $default = null) {
+    if (!empty($database->config)) {
+        $config = json_decode($database->config);
+    } else {
+        $config = new stdClass();
+    }
+
+    if ($key === null) {
+        return $config;
+    }
+
+    if (property_exists($config, $key)) {
+        return $config->$key;
+    }
+    return $default;
+}
+
+/**
+ * Update the configuration for this database activity.
+ *
+ * @param   stdClass    $database   The object returned from the database for this instance
+ * @param   string      $key        The name of the key to set
+ * @param   mixed       $value      The value to set for the key
+ */
+function data_set_config(&$database, $key, $value) {
+    // Note: We must pass $database by reference because there may be subsequent calls to update_record and these should
+    // not overwrite the configuration just set.
+    global $DB;
+
+    $config = data_get_config($database);
+
+    if (!isset($config->$key) || $config->$key !== $value) {
+        $config->$key = $value;
+        $database->config = json_encode($config);
+        $DB->set_field('data', 'config', $database->config, ['id' => $database->id]);
+    }
+}