Upgrade cheerio and resolve type errors

This helps resolve some type errors with `domhandler`. In addition, we convert
to the new way of using `htmlparser2` with `cheerio`.

package.json

@@ -58,7 +58,7 @@
     "blueimp-canvas-to-blob": "^3.28.0",
     "browser-encrypt-attachment": "^0.3.0",
     "browser-request": "^0.3.3",
-    "cheerio": "^1.0.0-rc.5",
+    "cheerio": "^1.0.0-rc.9",
     "classnames": "^2.2.6",
     "commonmark": "^0.29.3",
     "counterpart": "^0.18.6",
@@ -75,6 +75,7 @@
     "glob-to-regexp": "^0.4.1",
     "highlight.js": "^10.5.0",
     "html-entities": "^1.4.0",
+    "htmlparser2": "^6.1.0",
     "is-ip": "^3.1.0",
     "katex": "^0.12.0",
     "linkifyjs": "^2.1.9",