Prepare api.drupal.org for PHP 7

Problem/Motivation

Eventually, all drupal.org subsites will need to move to PHP 7. One of the (probably?) easiest to move should be api.drupal.org.

So, this issue is about preparing the api.drupal.org codebase so that it can move to PHP 7.

The codebase is at https://bitbucket.org/drupalorg-infrastructure/api.drupal.org/src/7.x-prod/ (and can be cloned from there).

Proposed resolution

Audit the api.drupal.org codebase for PHP 7 compatibility, and update it as necessary.

Remaining tasks

1. Clone the codebase and put it into PHPStorm or another IDE. Check for warnings/errors.

2. TBD.