Perf/issue show without edit

Implements loading the issue's edit form asynchronously on demand instead of each time an issue's show action is called. 

Especially for larger projects, with a lot of members and other defined fields, loading the edit form later speeds up the initial loading time considerably.

Beware: This pull request is not base on the master but on https://github.com/opf/openproject/pull/28. This was done because https://github.com/opf/openproject/pull/28 changes eager loaded objects and as such is a prerequisite for this pull request. 

The implementation is based on reusing ajaxAppend, a javascript function already part of OpenProject. ajaxAppend needed to first be extended in order for it to offer the flexibility needed. This extra work should pay of in the long run.