feat: resolve links and embeds in news body

[niccofyren]

Closes: #19

Tried to get a more complete solution in place, but hit a lot of undocumented(?) or unsupported(?) wagtail parts. Will make a separate draft PR with some of the leftovers too show theory.

What we get in this PR

• Links in body is resolved to html Wagtail would use in own frontend
• Embeds (including image embeds) in body is resolved to html Wagtail would use in own frontend

What a future and better version would deliver

• More sophisticated markup with absolute urls, srcset, etc
• More granular adjustments on links and other embed types
• Limited, but frontend supported, image positions/variants
• ... or alternatively we could deliver data as json array and convert to html on frontend side (I think this is my current favourite since it decouples backend and frontend code more)

[mfyll]

LGTM 🚀