Slow down the frequency of WordPress "heartbeat" calls in CiviCRM admin

Heartbeat is important to WordPress for a number of tasks - e.g. checking continued authentication whilst on a page and implementing edit locks on post/page edit screens - but it is less important on CiviCRM admin pages and consumes server resources. Reducing the frequency of calls minimises the impact on servers and can make CiviCRM admin pages more responsive.

PR here: https://github.com/civicrm/civicrm-wordpress/pull/214

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information