EmailPasswordSignInPage should use theme primary color for Sign In button.

[pbozzay]

Currently the EmailPasswordSignInPage uses a FormSubmitButton for the Sign In button UI. However, this does not accept a custom color or a context (in order to use the theme primary color). As a result this widget is not usable when the color indigo does not fit the rest of the UI being presented.

It seems like it would be better to allow a context argument so that the UI can use the parent theme.