Add filter for plugins to add support link to login form.

[StevenDufresne]

What?

Sometimes users have difficulty with their two-factor settings. In the "Having Problems?" section, we currently don't have a way to add links that are not providers.

This PR adds a filter called two_factor_login_support_links to allow plugins to provide non-provider support links.

Why?

We want to give users access to documentation that can help them with using 2fa or with recovering their account.

For example, it would allow us to do

Having Problems?
- Use your authenticator app
- Use a recovery code
- Recover your account

How?

It adds a filter inside the <ul> so HTML can be passed in like so:

add_filter( 'two_factor_login_backup_links', function($links) {
	return array_merge( $links, [ '<a href="#">My Link</a>' ] );
}, 10, 1 );

Testing Instructions

1. Add the filter mentioned above
2. With 2fa configured, try logging in
3. Expect to see the new link in the list.

Changelog Entry

Added - Filter named two_factor_login_backup_links to support adding links to login form.

[dd32]

Suggested change

	/*
	* Allow plugins to add links to the two-factor login form.
	*/
	$links = apply_filters( 'two_factor_login_support_links', $links );
	// Echo out the filtered links
	foreach ( $links as $link ) {
	echo wp_kses_post( $link );
	}
	/*
	* Allow plugins to filter the backup method links on the login form.
	*/
	$links = apply_filters( 'two_factor_login_backup_links', $links );
	// Echo out the filtered links
	echo implode( '', $links );

Given this filter only runs when there are backup providers, calling it a support links might be confusing for some, we can probably name it something like two_factor_login_backup_links?

There's probably an argument that this should be built prior to the if ( $backup_providers ) conditional to output filtered data even when there exist no backup methods for the given user?

I don't think we need to explicitly sanitize the HTML here either, it's either hard-coded HTML or from a PHP function that can do anything anyway.

[kasparsd]

This appears to be addressed in the follow-up commits.

[dd32]

These points remain:

Given this filter only runs when there are backup providers, calling it a support links might be confusing for some, we can probably name it something like two_factor_login_backup_links?

OR

There's probably an argument that this should be built prior to the if ( $backup_providers ) conditional to output filtered data even when there exist no backup methods for the given user?

I suspect the intention of the filter was actually to be used when the user has no backup provider, or, at least that others who would have a use for it would be in that group.

[kasparsd]

@StevenDufresne Was there anything else you wanted to include here or is this ready for merge?

If you don't get around to it, we'll extend the inline docblock for the filter to include documentation for the arguments (like here) and also add it to the relevant readme section.