Add assets for portico signin pages.

This adds some of the required assets for the portico signin pages such
as logos and validation signals.

static/images/checkbox-valid.svg

@@ -0,0 +1,5 @@
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" version="1.1" id="Layer_1" x="0px" y="0px" width="96px" height="96px" viewBox="21 0 75 75" enable-background="new 0 0 96 96" xml:space="preserve">
+<g>
+	<path fill="#52c2af" d="M49.844,68.325c-1.416,0-2.748-0.554-3.75-1.557L27.523,48.191c-1.003-1.002-1.555-2.334-1.555-3.75   s0.552-2.749,1.555-3.75c1.001-1.001,2.333-1.552,3.75-1.552s2.75,0.551,3.753,1.553l14.019,14.017L82.14,5.504   c0.989-1.468,2.639-2.345,4.412-2.345c1.054,0,2.075,0.312,2.956,0.902c2.424,1.631,3.07,4.934,1.439,7.361L54.25,65.98   c-0.892,1.316-2.312,2.162-3.895,2.314C50.17,68.315,50.01,68.325,49.844,68.325z"/>
+</g>
+</svg>

static/images/zulip-logo.svg

@@ -0,0 +1,6 @@
+<svg xmlns:svg="http://www.w3.org/2000/svg" xmlns="http://www.w3.org/2000/svg" width="100%" height="100%" viewBox="0 0 40 40" version="1.1">
+    <g transform="translate(-297.14285,-466.64792)">
+        <circle cx="317.14285" cy="486.64792" r="19.030317" style="fill:#444;stroke-width:1.93936479;stroke:#444"></circle>
+        <path d="m309.24286 477.14791 14.2 0 1.6 3.9-11.2 11.9 9.6 0 1.6 3.2-14.2 0-1.6-3.9 11.2-11.9-9.6 0z" style="fill:#fff;stroke:#fff"></path>
+    </g>
+</svg>