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

@@ -1,4 +1,4 @@
-class DonationsController < ApplicationController
+class Contributions::DonationsController < ApplicationController
   before_action :require_user_signed_in
 
   # GET /donations

app/controllers/contributions/projects_controller.rb

@@ -0,0 +1,8 @@
+class Contributions::ProjectsController < ApplicationController
+  before_action :require_user_signed_in
+
+  # GET /contributions
+  def index
+    @current_section = :contributions
+  end
+end