Refactor wp-signup.php

[enshrined]

I know it's been brought up in #31744 but I thought it deserved it's own ticket. wp-signup.php is a bit of a mess and a massive pain to customise.

I propose we refactor some of the code by doing the following:

• Move the functions out into wp-includes
• Add more hooks for customisation purposes
• Refactor PHP so it's easier to read
• I also think it'd be a good time to look at the markup as it seems quite inconsistent on this page.

I understand that ​https://make.wordpress.org/core/2011/03/23/code-refactoring/ says we shouldn't refactor for the sake of it, but I feel this would be for the good or WordPress multisite in the long run as this page really is a massive pain.

[joemcgill]

Related #31928

[sabernhardt]

I'm closing this as a duplicate so the conversation about refactoring these pages would continue in one place. If you have anything to add, please comment on #31928.

If you have specific hooks to propose, you could open new tickets about those.