We're currently using innerHTML in some places in the amp-story extension; we should instead use createElement/appendChild to mitigate future risks where we move from using static templates to dynamic ones including publisher-specified data.

• SVG icons (Use background image icons in amp-story #11610)
• extensions/amp-story/0.1/amp-story-page.js: loadingScreen./*OK*/innerHTML = LOADING_SCREEN_CONTENTS_TEMPLATE;
• extensions/amp-story/0.1/system-layer.js: this.root_./*OK*/innerHTML = TEMPLATE;