Use wpmu_validate_blog_signup() to validate site url and title

[neelakansha85]

Use wpmu_validate_blog_signup() to validate site url and title on Step 2 instead of my custom validation in order to comply with wordpress validations and reduce the need to maintain validation rules with changes to wordpress.

An important element to keep in mind is wpmu_validate_blog_signup() out of the box doesn't allow hyphen/dash in site url's and so will have to add a filter for it to allow those while being validated from Site Setup Wizard. Since if added as a filter similar to approach from https://wordpress.org/support/topic/how-to-allow-in-site-urls?replies=3 would allow dash to be used by users in any form in the site url's, which is not what we intend in Site Setup Wizard. Users should not be allowed to use dashes for site url's instead they are only used by SSW to separate them off from site category's.