Allow speakers to provide assets for their talk

[fcamblor]

When speakers have access to their talk editor space, we should allow them to provide assets for their talks

Assets should be :

• A creation date
• An URL
• A type : recording|slides|sources-repository|other
• (optionally) a platform (every type should have a supported platform list), for example, for type recording we'd have youtube|twitch|dailymotion

Once made available, those assets would be displayed on talk details screen (see #93)

[fcamblor]

Linked to #103

Would also be interesting to have slido link for Q&A

[fcamblor]

It would also be interesting if speakers were able to submit their social media contact infos, that way, we might be able to ease creation of platform-specific listing per events (see this bluesky discussion)

(once they submit their social media infos once, we might store it in localstorage, so that this information can be easily copy-pasted from one talk/event to another, without storing it at user-level in firestore)

[fcamblor]

It would also be nice to show URL and copy/pastable QRCode to open rating page of the talk