Improve mentions with NIP21

[Giszmo]

I love that Astral is explicit about mentions and allows me to edit them. NIP21 now lets you fix some awkwardness though:

• [ ] Allow me to remove all mentions - If I want to reply to a thread mentioning yet somebody else, I want to remove all reply-mentions first - not one by one.
• [ ] Allow me to add mentions in the mentions list without having to reference them in the content.
• [ ] Allow me to remove mentions without removing the reference in the content - nip21 allows that. The content changing when I click [x] on a mention is weird.