Declare forms to use method="POST" explicitly

Currently the login and register forms do not declare `method="POST"`. This throws warning with some browser plugins (for example LastPass), that complain that the form is not secure. LastPass in particular checks if the property `method="POST"` is set. Even without the property, Firefox posts the password data (so it is not a security issue). 
This pull requests adds `method="POST"` explicitly to form fields.