Adjust sponsorship layout (#1593)

* Fix SVG being black
    Adobe Illustrator SVGs have rendering problems on PTIO for unknown to
    me reasons. Inkscape works fine.
* Rewrite with plain Flexbox
    Nicer end-user experience than Bootstrap grid

_includes/sponsors/gold.html

@@ -1,13 +1,10 @@
-<div class="container">
-  <div class="row justify-content-center">
-    <div class="col-10 col-sm-8 col-md-4 col-lg-3">
-      <a href="https://safing.io/" target="_blank" rel="noreferrer">
-        <img
-        src="/assets/img/svg/sponsors/safing.svg"
-        title="Safing Privacy Network - Launching on Kickstarter January 2nd, 2020"
-        alt="Safing Privacy Network - Safing.io Logo"
-        class="img-fluid" />
-      </a>
-    </div>
-  </div>
+<div class="d-flex flex-wrap justify-content-center">
+  <a class="m-3" href="https://safing.io" target="_blank" rel="noreferrer">
+    <img
+      src="/assets/img/svg/3rd-party/sponsors/safing.svg"
+      data-theme-src="/assets/img/svg/3rd-party/sponsors/safing-dark.svg"
+      width="210rem"
+      title="Safing Privacy Network - Launching on Kickstarter January 2nd, 2020"
+      alt="Safing Privacy Network - Safing.io Logo" />
+  </a>
 </div>