when propagating an element, cross-site validate if we’re changing the status to enabled

[i-just]

Description

When propagating an element to a site, we only run cross-site validation on the changed custom fields (#13675). However, when the site status changes to enabled, we should make an exception and cross-site validate everything.

Related issues

#16505

steps to reproduce:

• have at least two sites
• have a section enabled for both; the propagation method can be left as “all”, but then the default status should be ’disabled'
• the section needs an entry type with at least one required custom field (plain text will suffice) that is set to be translatable for each site
• on siteA, create a new entry in that section, fill out only the title and fully save
• fill out the plain text field, enable the entry only for siteA & fully save
• now edit the entry for siteA again, toggle siteB to be enabled and fully save
• before this PR: the entry saves and is enabled on siteB now despite not having the plain text field filled out
• after this PR: the cross-site validation kicks in and lets you know that you should fill out the required fields before enabling the entry for siteB