Refactor sign in via Touchstone/Shibboleth

Only handle sign-in via Touchstone in custom sessions controller in a
`before_action` callback. Remove rendering login page for root path if
no user is signed in; redirect to `/login` instead to ensure the return
URL hits `before_action` callback for `sessions#new`.

Addresses #18
Setup controller for home page

By default, `devise` redircts to the root page after sign out. With the
root path configured to redirect to the login page, users will be uanble
to sign out if they signed in via Touchstone/Shibboleth. Since their
Shibboleth session has not been cleared by Rails, visiting the root path
will send the user back to `/login` which will re-create their `warden`
session since `REMOTE_USER` is still present in Apache. Setting up a
static page for the root path prevents the `warden` / `devise` session
from being created.