Merge branch 'fix-sanitize-dfn' into 'master'

sanitize: allow <dfn> tag

### In brief
* Add `<dfn>` tag to allowed tags list
  * `<dfn>` represents the defining instance of a term in HTML
  * More [information about `<dfn>` on the w3school's website](http://www.w3schools.com/tags/tag_dfn.asp )

### Example
This stops article content such as...
```
Indian tea harvests are divided up by <dfn>flush</dfn>.
```
...from getting turned into...
```
Indian tea harvests are divided up by .
```

See merge request !45