#JC-1869 Create additional permissions in plugins

Add ability to extend permissions in plugins. Add CREATE_QUESTION
permission for question-n-answer plugin. If plugin is installed and
enabled you can see new permission on the branch permissions page.
But editing isn't support now, so for next step need to remake
permission manager for more generic work with BranchPermission for
ability to manage plugins permissions too. Also add some todo labels
in the code where need to avoid code duplication.