Footnote titles not translatable

The `footnotes` extension generates HTML code like this:

```
<a href="#fnref-%s" class="footnoteBackLink" title="Jump back to footnote %d in the text.">&#8617;</a>
```

The string "Jump back to footnote %d in the text" is not translatable or customisable. Neither is the U+21A9 character ↩, which isn't ideal in some languages that aren't left-to-right.