Simplify Danger module

Most of the code in this module was more or less an implementation of insertAdjacentHTML, which all modern browsers support! I've simplified accordingly.

I don't have a good way to test whether this is faster or slower.

```
   raw     gz Compared to master @ 06e5fcc010d8b24c04ef14d31eae446aeab11189
     =      = build/JSXTransformer.js
     =      = build/jasmine.js
     =      = build/react-test.js
 -1854   -451 build/react.js
  -215    -71 build/react.min.js
```