fix(dialog): set css position: fixed to .md-dialog-container

Ensures the dialog will always be in the center of the screen even when the user scrolls vertically.

Author: Kiougar

src/components/dialog/dialog.scss

@@ -7,7 +7,7 @@ $dialog-padding: $baseline-grid * 3;
   display: flex;
   justify-content: center;
   align-items: center;
-  position: absolute;
+  position: fixed;
   top: 0;
   left: 0;
   width: 100%;