Admin: Add configuration setting 'disable_send_mail'

.travis.yml

@@ -102,6 +102,7 @@ script:
 #  - google-chrome-stable --version
 #  - whereis chromedriver
 #  - chromedriver --version
+  - echo "\$_configuration[\"disable_send_mail\"] = true;" >> app/config/configuration.php
   - cd tests/behat
   - pwd
   - travis_wait 45 ../../vendor/behat/behat/bin/behat -v

main/inc/lib/api.lib.php

@@ -8936,7 +8936,7 @@ function api_mail_html(
 ) {
     global $platform_email;
 
-    if ('1' === getenv('TRAVIS')) {
+    if (true === api_get_configuration_value('disable_send_mail')) {
         return true;
     }
 

main/install/configuration.dist.php

@@ -1450,6 +1450,9 @@ $_configuration['course_catalog_settings'] = [
 // Disable fields to add an attachment when creating and announcement.
 //$_configuration['disable_announcement_attachment'] = false;
 
+// Disable sending emails.
+//$_configuration['disable_send_mail'] = false;
+
 // KEEP THIS AT THE END
 // -------- Custom DB changes
 // Add user activation by confirmation email