Enhancements when clicking "Use a different account" on "Sign in" page

[cvan]

1. Clear LocalStorage: localStorage.clear()
2. Sign in with an account
3. Load the Sign-in page
   
4. Click the "Use a different account" link
5. Notice my email is prefilled, even though I want to sign in to a different account:
   
6. Refresh the page
7. Notice you are taken to the original Sign-in page, not the form I saw after clicking "Use a different account"
   

Observations:

1. Upon clicking "Use a different account," my previous email was filled in, without a label above the field (because the placeholder text is unfortunately being used as an label). (It's good to remember the email for things like the "Reset password" page, but when I explicitly want to sign in with a different account, we should clear that field.)
2. The email field was not autofocussed, so I had to click the field to change it.

[ckarlof]

Since @zaach worked on the cached credentials patch, he's going to take a look at this.