Add authorize route for MyYoast connection

[herregroen]

We want to have a new admin route to get an access token for MyYoast.

This route will be called from our settings page and should do the following:

• Ensure the current user has sufficient privileges to make the connection ( can manage_options ).
• Ensure the OAuth client has been configured ( has both a client ID and client secret ).
• Get the provider from the OAuth client ( see Create OAuth Client Class #12093 ).
• Redirect to the authorization url ( $provider->getAuthorizationUrl(); ).

Test instructions:
Success:

• Make sure the OAuth client has both a client id and secret stored.
• Visit this route as an admin.
• You should be redirect to yoast.com ( make sure the redirect happens, if you made up the client ID the page after the redirect is allowed to fail ).

No permissions:

• Make sure the OAuth client has both a client id and secret stored.
• Visit this route as an author.
• You should be redirect to the premium page where a warning is shown that you do not have sufficient privileges.

No configuration:

• Make sure the OAuth client does NOT have a client id and secret stored.
• Visit this route as an admin.
• You should be redirect to the premium page where a warning is shown that the connection to MyYoast has not yet been configured.

parent #12136
Needs #12093