Implements #150

Adds an entry inside moderation section that allows moderators to check
pending tasks and mark them as solved.

app/models/administrator_task.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+class AdministratorTask < ActiveRecord::Base
+  belongs_to :source, polymorphic: true
+  belongs_to :user
+
+  validates :source, presence: true
+
+  default_scope { order(created_at: :asc) }
+
+  scope :pending, -> { where(executed_at: nil) }
+  scope :done, -> { where.not(executed_at: nil) } 
+end