allow prepopulating the username when oauthing, refs CNVS-10001

you can already prefill the username on the regular login page by setting
?pseudonym_session[unique_id] ... this commit lets you do that when doing
oauth (carries it through the redirect)

test plan:
1. generate a valid oauth access url from your favorite 3rd party app,
   e.g. /login/oauth2/auth?client_id=...
2. tack on pseudonym_session[unique_id]=blah
3. go to the url
4. you should get redirected to the login page, with blah filled in as the
   username

Change-Id: I3203163fe02ff7e75d09dc91922851a8c5d34fb0
Reviewed-on: https://gerrit.instructure.com/31283
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>
Reviewed-by: Jeff Belser <jbelser@instructure.com>
Product-Review: Marc LeGendre <marc@instructure.com>