Refactor Participable

There are several changes to this module:

1. The use of an explicit stack in Participable#participants
2. Proc behaviour has been changed
3. Batch permissions checking

== Explicit Stack

Participable#participants no longer uses recursion to process "self" and
all child objects, instead it uses an Array and processes objects in
breadth-first order. This allows us to for example create a single
Gitlab::ReferenceExtractor instance and pass this to any Procs. Re-using
a ReferenceExtractor removes the need for running potentially many SQL
queries every time a Proc is called on a new object.

== Proc Behaviour Changed

Previously a Proc in Participable was expected to return an Array of
User instances. This has been changed and instead it's now expected that
a Proc modifies the Gitlab::ReferenceExtractor passed to it. The return
value of the Proc is ignored.

== Permissions Checking

The method Participable#participants uses
Ability.users_that_can_read_project to check if the returned users have
access to the project of "self" _without_ running multiple SQL queries
for every user.

1 files changed, 2 insertions, 1 deletions

diff --git a/app/models/project_snippet.rb b/app/models/project_snippet.rb
index 5fba6baa204..25b5d777641 100644
--- a/app/models/project_snippet.rb
+++ b/app/models/project_snippet.rb
@@ -7,5 +7,6 @@ class ProjectSnippet < Snippet
   # Scopes
   scope :fresh, -> { order("created_at DESC") }
 
-  participant :author, :notes
+  participant :author
+  participant :notes_with_associations
 end