'rails g controller Dashboard index'
redirect to dashboard after logging in
view/layouts/_sidebar: fix typo
css: float sidebar to the left
sidebar: display 'Users' link only for admins
seed: add default users

Later on, we should probably use Faker to populate regular users.
sidebar: add dashboard link
sidebar: add posts link
sidebar: move parent div tag to application view
css/html: basic site facelift
rearrange posts view

The most important change is the limiting of the @posts instance
variable in the case that the logged-in user is not an administrator;
administrators have access to all posts, but users only have access to
those posts that they themselves authored.
move common code to sessions helper

We already use these methods in the posts controller (and they were
rightly broken); moving these to the session helper brings them into
scope for the entire application (whose behavior is heavily dependent on
the logged in/out distinction, as is the case for the posts controller).
posts: add basic validations