feat: implement `PostEmbed`

[zeucapua]

Implemented a new PostEmbed component to be used wherever a Post with an embed is being rendered. There are multiple types an embed can be:

• [x] Images
• [ ] Videos
• [x] Quote Post
• [x] Quote Post w Media
• [x] Custom Feed Embed
• [x] User List (Mod List, etc.)
• [x] Starter Pack
• [x] External Link

Closes #12

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