consistent names for custom input IDs

[merlinoa]

I updated the required input id names to use a consistent format. The IDs are now:

• "sign_in_email"

• "sign_in_password"

• "sign_in_submit"

• "register_email"

• "register_password"

• "register_password_verify"

• "register_submit"

These IDs are used in the sign in modules and 3 of the JavaScript files. Please review to make sure that I did not miss updating any of the IDs.