Moves divider outside of link area

Divider was part of Sign Up / Sign Out link. This moves the divider
outside of the link.
Adds link to sign up text in body

"Sign up" text in body of new category page was not a link. This commit
makes it a link.
Changes divider to border for CSS-only handling

The divider was part of the sign in area links. This commit removes the
divider from the HTML and makes it a border instead of a pipe "|". This
leads to a slightly taller divider, but it is also more of an even
height with the text than the pipe was.
Fixes #99

Sets cleared cookie values in case cookie isn't cleared so that user is
actually signed out when they click Sign Out.
Disables inputs and button on sign in form after successful sign in.

After successfully signing in the sign in form inputs and button were
still active, which seemed counter-intuitive. This disables the inputs
and button after a successful sign-in.
Adds sign in with a different account link

- Adds link after signing in for "signing in with a different account".
- Adds parameter URL flag for forcing the user to sign out (by clearing
the cookie).
Adds password field placeholder text
Hides sign up form when user is signed in.

- Hides sign up form when user is signed in.
- Provides link to sign up another account (which logs user out).
Fixes #96 - modal overrides later modals.

Preview model text was updated on all modal classes, which affected the
submission modal too. This commit scopes the modal content query to the
preview and the submission modals, respectively.
If an access token exists, then they are really logged in.
Same pattern as new-category for new-service.
Merge pull request #100 from codeforamerica/ans-refactor

Ans refactor