Split out InterActiveAuthDialog

Into a component that does Interactive Auth and a dialog that
wraps it, so we can do interactive auth not necessarily in a
dialog.

As a side effect:
 * Put the buttons for each auth stage in the stage itself.
   Some stages don't have submit buttons (and it's very possible
   other stages may have other buttons entirely, like 'resend')
   so it makes more sense for the buttons to live in the stage
   components themselves. Plus it saves the slightly evil
   calling-functions-on-react-children thing we were doing (and
   indeed extending that to show the submit button at all).
 * Give all BaseDialogs a cross in the top right to cancel. They
   were all dismissable by clicking outside or pressing esc, so
   this adds a more visually obvious way of dismissing them. Plus,
   it means our InteractiveAuthDialog can have a way of canceling
   the whole operation separate from buttons for the individual
   stages.

src/components/views/dialogs/BaseDialog.js

@@ -17,6 +17,7 @@ limitations under the License.
 import React from 'react';
 
 import * as KeyCode from '../../../KeyCode';
+import AccessibleButton from '../elements/AccessibleButton';
 
 /**
  * Basic container for modal dialogs.
@@ -59,9 +60,23 @@ export default React.createClass({
         }
     },
 
+    _onCancelClick: function(e) {
+        e.stopPropagation();
+        e.preventDefault();
+        this.props.onFinished();
+    },
+
     render: function() {
         return (
             <div onKeyDown={this._onKeyDown} className={this.props.className}>
+                <AccessibleButton onClick={this._onCancelClick}
+                    className="mx_Dialog_cancelButton"
+                >
+                    <img
+                        src="img/cancel.svg" width="18" height="18"
+                        alt="Cancel" title="Cancel"
+                    />
+                </AccessibleButton>
                 <div className='mx_Dialog_title'>
                     { this.props.title }
                 </div>