Change donations to contrbutions, add tabbed nav

Introduces components for tabbed navigation and adds a tab menu and item
for non-financial contributions to the donations/contributions page.

app/controllers/contributions/donations_controller.rb

@@ -0,0 +1,10 @@
+class Contributions::DonationsController < ApplicationController
+  before_action :require_user_signed_in
+
+  # GET /donations
+  # GET /donations.json
+  def index
+    @donations = current_user.donations.completed
+    @current_section = :contributions
+  end
+end