Add post comments

[scudo005]

Requirements:

1. Must be FOSS
2. Must be self-hostable by someone else if necessary (no centralized services)
3. Must be able to delete spam comments
4. Must allow the usage of <iframe> for embedding (since GH Pages is static pages only)

e.g. someone running a SQL database and a super simple page for the comments.

[scudo005]

Note for future self:

• No, it's not a guestbook
• Every post has its own comments
• Only posts have comments