Remove route visitors/_track; bootstrap drupal

Problem/Motivation

Visitors use a Controller and the Drupal routing system to log their visits. Fully bootstrapping Drupal to log visitor data is unnecessary.

Steps to reproduce

Proposed resolution

Create a file in the root of the repo: visitors.php
Inside visitors.php, bootstrap Drupal, like done in the statistics module.

Remaining tasks

1. Create visitors.php
2. Update javascript snippet with visitors.php path

User interface changes

API changes

Data model changes