Miscellaneous changes before Scheduler 8.x-1.2

To explain the above commit: our Travis test build was getting the Rules module code from github - https://github.com/fago/rules - because when we started using Travis this was an active repository with the latest Rules developments before they were committed to drupal.org. This was very useful when developing our Scheduler_Rules_Integration sub-module and its tests. However, this github repository is no longer being maintained and has slipped way behind the drupal.org version, so it is now preferrable to get the code from https://git.drupalcode.org/project/rules

One immediate benefit is that the number of Legacy Deprecation Notices listed in our Travis test log has dropped, due to some recent fixes in Rules code (Core 8.6 was 777 down to 642, core 8.7 was 1750 now down to 1615, and core 8.8 was 2214 now down to 2079). These are deprecation warnings created by Rules and Devel Generate, not by Scheduler's own code.

Change .travis.yml to add a PHP7.3/Core 8.8 combo to the build matrix.

Make PHPCS ignore all files which start with _ignore (as this helps to keep things cleaner).
Fail the Travis build if warnings are found by PHPCS (previously only coding standard errors would fail the build).

use Drupal\Tests\Traits\Core\CronRunTrait in our SchedulerBrowserTestbase so we can remove cronRun() which was added as a temporary measure while we were still running at Core 8.2

In tests, move the creation of a non-scheduler content type into SchedulerBrowserTestBase

Added extensions to phpcs.xml.dist following this Coder change #3074176: Allow extensions to be overridden in contrib phpcs.xml file

When running devel generate via drush there is (currently) no way to specify the scheduler publishing/unpublishing percent parameters. Hence drush genc 1 -v with verbose option on, shows:

 [info] Undefined index: scheduler_publishing scheduler.module:358
 [info] Undefined index: scheduler_unpublishing scheduler.module:359

Fix will follow.

The travis test at core 8.9 and PHP7.0 started failing recently with error

ParseError: syntax error, unexpected ',', expecting ']'
core/lib/Drupal/Core/Database/Database.php:571

Line 571 is

    [, $module, $module_relative_namespace] = explode('\\', $namespace, 3);

Core 8 requires PHP7.0.8 minimum https://www.drupal.org/docs/8/system-requirements/php-requirements
Specifying 7.0 in .travis.yml loads 7.0.3 but there's no real need to test with 7.0.8 as we can simply change to PHP7.1.

Coder 8.3.9 has a new sniff which returns

scheduler/scheduler.module
----------------------------------------------------------------------
FOUND 0 ERRORS AND 1 WARNING AFFECTING 1 LINE
----------------------------------------------------------------------
 645 | WARNING | Exceptions should not be translated
     |         | (DrupalPractice.General.ExceptionT.ExceptionT)
----------------------------------------------------------------------

Scheduler 8.x-1.2 is now released