Resolve frontend/backend contradiction for home visibility embeds

What does this MR do?

This MR resolves a contradiction between frontend and backend code that didn't allow posts set to Home visibility to be embedded, causing the Generate Embed Code popup to load forever.
In order to do this, the check from the frontend is copied to the backend.

The contradiction in question:

• Backend
• Frontend

This issue also exists on upstream:

• Backend
• Frontend

CC @kakkokari-gtyih

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