Change `hypothesis-asset` attribute to `data-hypothesis-asset`

Adding a custom attribute without a `data-` prefix could potentially
cause headaches for publishers running their code through HTML
validation tools.

src/annotator/index.js

@@ -78,7 +78,7 @@ function init() {
 
     // Remove all the `<link>`, `<script>` and `<style>` elements added to the
     // page by the boot script.
-    const clientAssets = document.querySelectorAll('[hypothesis-asset]');
+    const clientAssets = document.querySelectorAll('[data-hypothesis-asset]');
     clientAssets.forEach(el => el.remove());
   });
 }

src/boot/boot.js

@@ -36,7 +36,7 @@ const commonPolyfills = [
  * @param {HTMLElement} el
  */
 function tagElement(el) {
-  el.setAttribute('hypothesis-asset', '');
+  el.setAttribute('data-hypothesis-asset', '');
 }
 
 /**

src/boot/test/boot-test.js

@@ -69,13 +69,13 @@ describe('bootstrap', function () {
 
   function findAssets(doc_) {
     const scripts = Array.from(
-      doc_.querySelectorAll('script[hypothesis-asset]')
+      doc_.querySelectorAll('script[data-hypothesis-asset]')
     ).map(function (el) {
       return el.src;
     });
 
     const styles = Array.from(
-      doc_.querySelectorAll('link[rel="stylesheet"][hypothesis-asset]')
+      doc_.querySelectorAll('link[rel="stylesheet"][data-hypothesis-asset]')
     ).map(function (el) {
       return el.href;
     });
@@ -102,7 +102,7 @@ describe('bootstrap', function () {
         'link[type="application/annotator+html"]'
       );
       assert.ok(sidebarAppLink);
-      assert.isTrue(sidebarAppLink.hasAttribute('hypothesis-asset'));
+      assert.isTrue(sidebarAppLink.hasAttribute('data-hypothesis-asset'));
       assert.equal(sidebarAppLink.href, 'https://marginal.ly/app.html');
     });