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)