Lists (#5703)

* Add structure for lists

* Add list timeline streaming API

* Add list APIs, bind list-account relation to follow relation

* Add API for adding/removing accounts from lists

* Add pagination to lists API

* Add pagination to list accounts API

* Adjust scopes for new APIs

- Creating and modifying lists merely requires "write" scope
- Fetching information about lists merely requires "read" scope

* Add test for wrong user context on list timeline

* Clean up tests

db/migrate/20171114231651_create_lists.rb

@@ -0,0 +1,10 @@
+class CreateLists < ActiveRecord::Migration[5.1]
+  def change
+    create_table :lists do |t|
+      t.references :account, foreign_key: { on_delete: :cascade }
+      t.string :title, null: false, default: ''
+
+      t.timestamps
+    end
+  end
+end

db/migrate/20171116161857_create_list_accounts.rb

@@ -0,0 +1,12 @@
+class CreateListAccounts < ActiveRecord::Migration[5.1]
+  def change
+    create_table :list_accounts do |t|
+      t.belongs_to :list, foreign_key: { on_delete: :cascade }, null: false
+      t.belongs_to :account, foreign_key: { on_delete: :cascade }, null: false
+      t.belongs_to :follow, foreign_key: { on_delete: :cascade }, null: false
+    end
+
+    add_index :list_accounts, [:account_id, :list_id], unique: true
+    add_index :list_accounts, [:list_id, :account_id]
+  end
+end