Parse error when issue/pull has old huboard config lines

If a pull has something like the following as it's body we get parse errors. This is because of the code in `issue_helpers.js` that tries to find the `conf_begin` and `conf_end`. Since `conf_end` is found before `conf_begin` we end up with an empty `config_str` which causes a JSON parse error. It appears that the end result is that we continue to add more config sections that in the end are not used.

```
"This is some pull with a discussion in it.

<!---
@huboard:{"order":8.0999323128772e-06}
-->

<!-- TRESTLE
{"columnWeight":-881.5,"milestoneWeight":-671}
-->
<!-- TRESTLE
{"columnWeight":-637,"milestoneWeight":210}
-->"
```


<!-- TRESTLE
{"columnWeight":4,"milestoneWeight":969}
-->
<!-- TRESTLE
{"columnWeight":-951.5,"milestoneWeight":674}
-->
<!-- TRESTLE
{"columnWeight":0,"milestoneWeight":384}
-->
<!-- TRESTLE
{"columnWeight":0,"milestoneWeight":-261}
-->