Feature parent access

[Exelord]

Assumption

Add a possibility to customize accessible_for with parent_access flag to allow user determine if children of accessible resources should be also accessible.

Resource.accessible_for(current_user, { parent_access: true })
#                     (GRANTED) project1
#                                  |
#                                  |
#              (GRANTED) project2   project4 (GRANTED cuz it's a parent of granted resource)
#                           |          |
#                           |          |
#                           |       project5 (NOT granted - it's not a child of granted resource)
#                           |
#                           |
#                        project3 (member role - GRANTED)

Todo

• Improve parent_access query (union -> join)