Move icons/ to assets/

There's a default alias for icons/ in apache,
https://www.electrictoolbox.com/apache-icons-directory/

proxy/static/assets/arrowhead-right-12.svg

@@ -0,0 +1,4 @@
+<!-- This Source Code Form is subject to the terms of the Mozilla Public
+   - License, v. 2.0. If a copy of the MPL was not distributed with this
+   - file, You can obtain one at http://mozilla.org/MPL/2.0/. -->
+<svg xmlns="http://www.w3.org/2000/svg" width="12" height="12" viewBox="0 0 12 12"><path fill="context-fill" d="M9 6a1 1 0 0 0-.293-.707l-3-3a1 1 0 0 0-1.414 1.414L6.586 6 4.293 8.293a1 1 0 0 0 1.414 1.414l3-3A1 1 0 0 0 9 6z"/></svg>