Feature knowledgebase

Modified and renamed the redmine_questions plugin into a knowledgebase plugin.

Features:
Device categorisation fields for kb article
Related issues for article
Convert issue into kb article

Using these article enhancements (and the previously-existing tags feature), I will now amend the did_you_mean_qa plugin to perform a more sophisticated search for kb articles. 

Known problems:
Cannot override redmine localisations as desired, so some of the terminology is wrong (message rather than article, for example).