Change BudgetInvestmentStatus to Milestone::Status

Generalize the BudgetInvestmentStatus model to Milestone::Status so it
is not specific to budget investments, but can be used for any entity
which has milestones. This is in preparation to make the Milestone
model polymorphic and usable by entities other than budget investments.

app/controllers/admin/milestone_statuses_controller.rb

@@ -0,0 +1,51 @@
+class Admin::MilestoneStatusesController < Admin::BaseController
+
+  before_action :load_status, only: [:edit, :update, :destroy]
+
+  def index
+    @statuses = Milestone::Status.all
+  end
+
+  def new
+    @status = Milestone::Status.new
+  end
+
+  def create
+    @status = Milestone::Status.new(status_params)
+
+    if @status.save
+      redirect_to admin_milestone_statuses_path,
+                  notice: t('admin.statuses.create.notice')
+    else
+      render :new
+    end
+  end
+
+  def edit
+  end
+
+  def update
+    if @status.update(status_params)
+      redirect_to admin_milestone_statuses_path,
+                  notice: t('admin.statuses.update.notice')
+    else
+      render :edit
+    end
+  end
+
+  def destroy
+    @status.destroy
+    redirect_to admin_milestone_statuses_path,
+                notice: t('admin.statuses.delete.notice')
+  end
+
+  private
+
+  def load_status
+    @status = Milestone::Status.find(params[:id])
+  end
+
+  def status_params
+    params.require(:milestone_status).permit([:name, :description])
+  end
+end