Re #1207 - My Europeana: Login, New Password, Register

* adjusted controller so that login error is returned in the GET request, not the POST
* adjusted the static strings for requested-action
* added a static string for the invalid credentials message that now refers to a message.property key rather than hard coded text
* adjusted forms so that they now use requested-action as the hidden field to indicate form action
* adjusted forms so that any messages appear just above the form fieldset