InvalidArgumentException thrown when Piwik URL doesn't end in a slash

When the user enters a valid Piwik URL that doesn't end in a slash, the name of the script is appended without checking if the resulting URL is valid, resulting in the following crash:

InvalidArgumentException: Unable to parse URI: http://localhost:8000piwik.php in GuzzleHttp\Psr7\Uri->__construct() (line 71 of /home/pieter/v/joinup/vendor/guzzlehttp/psr7/src/Uri.php).

Let's autofix the URL like we did in the Drupal 6 & 7 versions of the module, see #1776938: Piwik HTTP URL Validation (trailing slash).