Wordpress - Plugins must have

https://en-ca.wordpress.org/plugins/remove-http/

Description
Remove HTTP is a plugin that automatically scans and removes both http: and https: protocols from all links. This helps resolve websites that are having “mixed content warnings” which is when the website has assets (images, JavaScript, and CSS) loading both HTTP and HTTPS.
No changes to the links are made in the database. Simply install and activate the plugin and the changes will be immediate. Links that have http:// or https:// will only have //, making them protocol relative URLs. Below is a before and after example.