moves issue tree row rendering into helper

The partial was using content_tag methods anyway so readability was not improved by using a partial vs. a helper. Additionally the partial contained a lot of non view related ruby code. That and the fact that partials are expensive lead to moving it into a helper.
switches to join concatenation (as it was faster in my test)
uses a journal cache to render the history

See the comment for some details.

We should think about moving this into a different place like the
controller or the User.current. Doing so would provide us with a request
cache that can be reused for multiple purposes.

However this might cause serious memory issues.
removed join for always -one element- array, changed this to string
Merge pull request #26 from opf/perf/issue_show_history

Perf/issue show history