Groups active method
Groups should have "active" method, because they can be assigned issues to, and are treated in many ways as users.
Some plugins like https://github.com/Restream/redmine_reminder use it, and even cause error as described here
Even though it could be resolved as a plugin my opinion is that groups.rb model should have active method, simply because they
already have status attribute...
So proposed is to add active? method, as provided in patch file.