[Fix bug 1033298] Enable markdown support in new vouching interface.

`markdown` filter adds another `<p>` and that's ugly
![screenshot-127 0 0 1 8000 2014-07-03 11-37-49](https://cloud.githubusercontent.com/assets/584352/3467185/65375502-028d-11e4-9719-30612f41bc02.png)

How about removing "reason" can displaying user's text directly, like this?
```
{% if not vouch.description %}
<p>{{ _('Legacy vouch.') }}</p>
{% else %}
{{ vouch.description|markdown  }}
{% endif %}
```

Which results in this?
![screenshot-127 0 0 1 8000 2014-07-03 11-40-08](https://cloud.githubusercontent.com/assets/584352/3467208/adef7fa4-028d-11e4-822a-8c0998523427.png)