Tweaks to login and sign up form in alpha

* Make corners rounded for inputs
* Center links underneath form
* Hide specific content to focus on primary purpose of form

See also:  I175e9f515d962f9078845f25f9b96079819444c6

Change-Id: I16ac04f467d4f16977546ec276f89b1ac871114e
Merge "Tweaks to login and sign up form in alpha"