Revert "posts controller: allow admins to destroy posts"

This reverts commit 12b9f02cb9bad5ec0dcf1f6a3b1fef39b221c729.

This prevented regular users from destroying their own posts.
correct_user: use current_user.id

We only check for correct_user if a user is already logged in, which
prevents `current_user` from being nil (it's safe to do it this way).
post: use field 'user_id'

This follows the Rails convention of naming a foreign key field as
<table_id>.
schema: set default role_id

The spirit of this change was actually defined back in
f35d0708385f27e7bbd8040537cb5c31725c809e, but the db was not recreated
and re-migrated properly --- hence the schema was incorrect. After
completely resetting the DB, this line was automatically added in by
Rails.
user: make association with posts

This should have been included when we first created the "Post" model.
correct_user: use unique definition across controllers

This way, we define what a "correct" user is, depending on what model we
are concerned with. If this kind of definition becomes too repetitive,
we could probably put it back into the sessions controller and pass the
context into it, so that we have a big case/when/else statement to take
care of the different model contexts. However, for our purposes,
redefining it per model is sufficient.