SakesHelperの定数まわりをリファクタ

[momocus]

• start_year_limitメソッドを定数にした
• buttom_buttoleメソッドを、Sakeモデルの定数にした

関連 #594
（まだSakesControllerからIncludeを取り除けてない。プルリク大きくなりそうだから、いったんレビューリクエスト出すわ）

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.03 ⚠️

Comparison is base (2e9a3cc) 95.49% compared to head (7d034d1) 95.46%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #600      +/-   ##
==========================================
- Coverage   95.49%   95.46%   -0.03%     
==========================================
  Files          16       16              
  Lines         311      309       -2     
==========================================
- Hits          297      295       -2     
  Misses         14       14              

Impacted Files	Coverage Δ
app/controllers/sakes_controller.rb	97.97% <100.00%> (ø)
app/helpers/sakes_helper.rb	100.00% <100.00%> (ø)
app/models/sake.rb	100.00% <100.00%> (ø)

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