Forbid discussion author from deleting the first post of a discussion

## Feature Request

**Is your feature request related to a problem? Please describe.**

On my forum quite often users delete the first post of a discussion when they instead want to delete the whole discussion. I can tell that this is the case because they often write "I'm going to delete the discussion", but then they don't, probably because they don't find the right button or settle with the first "delete" they see.

**Describe the solution you'd like**

Deleting the first post of a discussion shouldn't be allowed. I don't think it makes sense, I can't think of a legitimate case in which it's a good idea to do that. And it would solve the above mentioned problem.

For context, this was proposed also in flarum/issue-archive#306 by Franz. It might also be related to the implementation of flarum/framework#90, which is however quite old so I don't know if that code is actually still around.

No files, tests, or entry points are identified. Start by locating the authorization and deletion flow for discussion posts, then read flarum/issue-archive#306 and flarum/framework#90 for prior context; done means the discussion's author cannot delete its first post.

Written by the indexing model from the issue text.