NIP-09 Deletion Event Improvements

[vitorpamplona]

1. Simplifies the wording
2. Removes duplicated text
3. Removes behavior ideas for clients and relays.
4. Adds k-tag support
5. Adds a-tag's time limitation (deletes everything up to the event's created at).

Read here

Supercedes @basantagoswami's PR #1069

Fixes: #779

[vitorpamplona]

Also, I was today years old when I realized that the old text allowed a content edit: "The content field MAY also be used to replace the deleted events' own content, although a user interface should clearly indicate that this is a deletion reason, not the original content."

I got rid of this since no one is using Delete events to replace content.

[vitorpamplona]

Can we merge this? @fiatjaf