Chore: Add default value for label color (#981)

db/migrate/20200625154254_add_default_value_to_color.rb

@@ -0,0 +1,11 @@
+class AddDefaultValueToColor < ActiveRecord::Migration[6.0]
+  def up
+    Label.where(color: nil).find_each { |u| u.update(color: '#1f93ff') }
+
+    change_column :labels, :color, :string, default: '#1f93ff', null: false
+  end
+
+  def down
+    change_column :labels, :color, :string, default: nil, null: true
+  end
+end