unify add site button, access, redirects in cp and user page #3224

#3224

#### Changes
 * New function _vsite_add_site_link() returns build array for a link or link item.
 * Replaced 2 regular build instantiations with the new function (CP Subsite link, User Page)
 * Updated vsite access function to include warning message about custom domains

#### QA todo

##### A. Add subsite link correct
1. Log in as admin on a department site with a custom domain.
1. **Test**: the "Add Subsite" button in the CP should link to the sitewide URL

##### B. Better error message on custom domain site register
1. Log in as an admin on a site with a custom domain.
1. Go to site/register.
1. **Test**: The error message should say you can't add new sites to custom domains.