Eliminate repetitive code in signin, signup, and signout

[adalinesimonian]

There exists a number of passages of code in signin, signup, and signout which are repeated throughout with minor differences.

• oidc/signout.js lines 43-45, 57-59, 73-75, 81-83
• routes/signin.js lines 78-85 and routes/signup.js lines 67-74

[christiansmith]

Just to give everyone else context, we took a crack at this and Passport complicates the matter because of the way it monkey patches http.req. The code we ended up with on the first pass seemed dirtier than having some repetition so we backed out the changes. Ugh.