Add Referable concern

app/models/concerns/referable.rb

+# == Referable concern
+#
+# Contains functionality related to making a model referable in Markdown, such
+# as "#1", "!2", "~3", etc.
+module Referable
+  extend ActiveSupport::Concern
+
+  # Returns the String necessary to reference this object in Markdown
+  #
+  # from_project - Refering Project object
+  #
+  # This should be overridden by the including class.
+  #
+  # Examples:
+  #
+  #   Issue.first.to_reference               # => "#1"
+  #   Issue.last.to_reference(other_project) # => "cross-project#1"
+  #
+  # Returns a String
+  def to_reference(_from_project = nil)
+    ''
+  end
+
+  module ClassMethods
+    # The character that prefixes the actual reference identifier
+    #
+    # This should be overridden by the including class.
+    #
+    # Examples:
+    #
+    #   Issue.reference_prefix        # => '#'
+    #   MergeRequest.reference_prefix # => '!'
+    #
+    # Returns a String
+    def reference_prefix
+      ''
+    end
+  end
+
+  private
+
+  # Check if a reference is being done cross-project
+  #
+  # from_project - Refering Project object
+  def cross_project_reference?(from_project)
+    if Project === self
+      self != from_project
+    else
+      from_project && project && project != from_project
+    end
+  end
+end