HTML import: Replace cheerio with jsdom to simplify contentcollector

Cheerio provides jQuery-like objects but they wrap DOM Node-like
objects that are not 100% API compatible with the DOM spec. Because of
this, contentcollector, which is used in browsers and in Node.js
during HTML import, has until now needed to support two different
APIs. This commit modifies HTML import to use jsdom instead of cheerio
and simplifies contentcollector.

CHANGELOG.md

@@ -1,3 +1,22 @@
+# Next Release
+
+### Compatibility changes
+
+* For plugin authors:
+  * Etherpad now uses [jsdom](https://github.com/jsdom/jsdom) instead of
+    [cheerio](https://cheerio.js.org/) for processing HTML imports. There are
+    two consequences of this change:
+    * `require('ep_etherpad-lite/node_modules/cheerio')` no longer works. To
+      fix, your plugin should directly depend on `cheerio` and do
+      `require('cheerio')`.
+    * The `node` context argument passed to the `collectContentImage` hook is
+      now an
+      [`HTMLImageElement`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLImageElement)
+      object rather than a Cheerio Node-like object, so the API is slightly
+      different. See
+      [citizenos/ep_image_upload#49](https://github.com/citizenos/ep_image_upload/pull/49)
+      for an example fix.
+
 # 1.8.14
 
 ### Security fixes