posts view: limit visibility of "Author" column

It only makes sense to display authorship information for admins. For
regular users, they already only see their own posts.
post controller: new post: automatically set authorship

Author field is set to logged-in user's user id.
post: view: new: remove redundant link

This link path is already visible in the sidebar as "Posts".
post: _form template: disable "author" field

This field is automatically set by the post controller (post#new), so it
does not make sense to put it here. Even in the case of administrators,
it does not make sense to change an existing post's authorship to
another user.
new post: enable 'published' field by default
posts: only logged in users can perform CRUD operations
posts controller: allow admins to destroy posts
post creation: put in user ID for authorship

This is necessary because we do not have the 'Author' field in the form
for new post creation; this means that the application will not receive
the 'Author' parameter when we click on "Create Post".

Also, we remove the 'author: current_user.id' value for the @post
instance variable @post, because we do not need to have that variable at
all in the form before we click on "Create Post".
posts: index: find posts by correct foreign key

The current_user.id should be compared to a post's 'author' field, not
its own 'id' field.