Let admins delete or edit any post

Previously: admins could not delete or edit posts that they didn't own
Problem: admins should be able to do so (especially for spam or offensive posts)
Solution: if a user is an admin, delete or update posts using Post.find instead of current_user.posts
This should just be `Post`. Remember you're calling `.find` on it later so that will be like `Post.find`, which is better than `Post.all.find`.