Fix CMS page plain text fields not being translated

[SamueleMartini]

Summary

• Fix title, content_heading, meta_title, meta_keywords, meta_description not being translated on CMS pages
• TranslateParsedContent::execute() for string input now translates the text via Translator before handling widget directives
• Add identifier (URL Key) to CMS page allowed translation fields
• Fix url_key slug sanitization check to use correct field name (identifier)

Test plan

• Translate a CMS page and verify title, content heading, meta fields are translated
• Verify URL key is translated and properly slugified
• Verify content with Page Builder HTML still translates correctly
• Verify widget directives in plain text fields are handled correctly

[dadolun95]

Lgtm