Move ProjectInvolvementSpec outside de spec/fast/ directory (because it
loads rails..)
Allow IssueUserVisibility service to deal with role constraints
Simplify IssueUserVisibility specs
Refine the way IssueUserVisibility checks viewing permissions on a specific issue

Basically it includes the project_id of the issue so that issues'
authoried_viewers can be fetched globally for listing needs. Else if we
had just the role ids at the issue level, the mask wouldn't be useable
directly for cross-issues needs (activity, issues/index, etc.)
Add roles+projects masks to Issue.visible_condition

(symetric to what Issue#visible? does through IssueUserVisibility in
previous commits, but for multiple-issues or cross-projects requests)
Replace ProjectInvolvement#potential_involved_teams with #potential_involved_roles

This decouples the plugin from the redmine_organizations plugin for now
and allows us to have a first version based on roles, even if the
underlying patches to Issue#visible? and Issue.visible_condition support
many more types of requests (filtering by user, group, org, or
role+project)
Add views for issue visibility management at issue level (NOT FUNCTIONAL!)
Add UI for managing default settings regarding Issue#authorized_viewers in role's form
Add some more informations in the README