Move all actions and filters outside constructor functions

We have a lot of actions and filters inside constructor functions. Example:

https://github.com/Automattic/WP-Job-Manager/blob/master/includes/class-wp-job-manager-ajax.php#L14

This makes it hard to, say, remove an action and use your own instead. We should move them all out of `__construct`.

Start by reviewing includes/class-wp-job-manager-ajax.php, especially the constructor around the linked example, and identify the actions and filters registered there. Trace the other constructor functions that register hooks, then verify that hook registration remains available for replacement or removal without changing existing behavior.

Written by the indexing model from the issue text.