Update markdown reference to external issues

1. Issue may be not only in jira.
2. Rewrite method for support different external issue trackers
I changed this line to
```ruby
url = url_for_issue(identifier)
```
or it showed no url in dashboard like this

```html
<a class="gfm gfm-issue " href title="Issue in Redmine">33944</a>
blablabla
<a>
```