Allow textboxes to have width set, default to 2/3

For entering 2 or 3fa codes, we want a textbox that’s just over 6 characters
wide.

To do this, a width can now be passed to the textbox macro. The possible widths
are the same as those provided by GOV.UK Elements, and in the same format (eg
1-4, 1-2, 2-3…)

This commit also adds a new width (5em) which is suitable for 3fa codes, and
adds it to the verify page.

app/assets/stylesheets/app.scss

@@ -30,3 +30,11 @@
 a:visited {
   color: $link-colour;
 }
+
+.form-control-5em {
+  width: 100%;
+
+  @include media(tablet) {
+    width: 5em;
+  }
+}