Purify the entry title and body on the server side

[Tiendil]

• Frontend-side purification became too complex and hard to maintain.
• It may be more efficient (traffic, speed) to transfer already sanitized content to the frontend.
• By keeping purification on the server side, we can ensure that all clients receive clean and consistent content, regardless of their individual capabilities or limitations.
• It is also a more secure approach, as it reduces the risk of malicious content being executed on the client side.

Notes:

• We already have a partial sanitization on the backend via feedparser (docs), however, we need more control over the purification process and also we may need a unpurified version of the content for some use cases (e.g., integration plugins).
• We may want to store 2 versions of the entry body and title: raw and sanitized+normalized.