Issue title: set_current_user hook not working when Yoast is active
 Issue body: ### What did you expect to happen?

Action added to the `set_current_user` hook to fire when Yoast is enabled.

### What happened instead?

Action not being nicole98@example.com.

### How can we reproduce this behavior?

1.
Fresh install of WP with Yoast and twentysixteen activated.
No other plugins.

2.
Add this to functions.php
```
add_action('set_current_user', 'setcurr_function');
function setcurr_function() {
	error_log('This will not be logged when YOAST is activated');
}
```
```
add_action('init', 'init_function');
function init_function() {
	error_log('This will be logged both with and without YOAST');
}
```
3.
Run site, check log

4.
Disable YOAST

5.
Run site, check log

### Technical info
* WordPress version: 4.6.1
* Yoast SEO version: 3.7.1
 Comments: 
 Comment 0: Any plugin at all which calls any user-related check at all on load will cause such situation, since plugins are loaded before themes. Due to this timing it's not practical to use such hook via theme's code.
{'source': 'GithubIssues', 'repo_name_with_owner': 'Yoast/wordpress-seo', 'issue_id': 186583144, 'repo_stars': 1707.0}