Login page - Sign In button should not be disabled

Having the Sign In button be disabled is causing problems now that the browser can remember and prepopulate the username/password. For example:

When using the autofill feature of the AML UI, I typed in a username, clicked it from the autocomplete box and had the password autofill. I then tried to click the Sign In button and it does nothing. If I place the cursor in the password box and hit the enter key, I will get signed in normally. I have only reproduced this on Firefox.

The problem is that there is no reliable event to detect autofill.
So, instead of ever disabling the Sign In button, we just need to make sure the UI validates properly when the button is clicked.