Fix #170: stray bar at bottom of /new page

#139 removed inline display:block from paste.html's created banner and
compensated with a global .created-banner { display:block } override.
That override also hit the hidden placeholder banner on /new, whose
base rule is display:none, making an empty padded bar render above the
action bar. Scope the override to .created-banner.show and mark only
the paste-view banner with the show class.

internal/web/static/app.css

@@ -786,4 +786,4 @@ button[type="submit"]:focus-visible,
 .mt18 { margin-top: 18px; }
 .toggle-inline { display: inline-flex; }
 .wrap-normal { word-break: normal; overflow-wrap: break-word; }
-.created-banner { display: block; }
+.created-banner.show { display: block; }