Rename concern to Shardable

A shard is now the proper name for a repository storage, so Shardable
is an acceptable name for defining the repository shard.

app/models/concerns/repository_on_shard.rb → app/models/concerns/shardable.rb

 # frozen_string_literal: true
 
-module RepositoryOnShard
+module Shardable
   extend ActiveSupport::Concern
 
   included do

app/models/pool_repository.rb

 # frozen_string_literal: true
 
 class PoolRepository < ActiveRecord::Base
-  include RepositoryOnShard
+  include Shardable
 
   has_many :member_projects, class_name: 'Project'
 

app/models/project_repository.rb

 # frozen_string_literal: true
 
 class ProjectRepository < ActiveRecord::Base
-  include RepositoryOnShard
+  include Shardable
 
   belongs_to :project, inverse_of: :project_repository