Instead of deleting them entirelly, in many cases it might be better just to mark user accounts and groups as disabled. This commit contains following relevant changes:
* Updates in SQL data model for users and groups
* New base views, base form and form templates for generic changes of items
* Support for enable and disable actions in item forms and views (show,list)
* Support for enabled/disabled accounts in authentication controllers (disabled accounts will not permit login)

(Redmine issue: #3752)