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/views/shared/_tabnav_contributions.html.erb

@@ -0,0 +1,8 @@
+<%= render TabnavLinkComponent.new(
+  name: "Donations", path: contributions_donations_path,
+  active: current_page?(contributions_donations_path)
+) %>
+<%= render TabnavLinkComponent.new(
+  name: "Projects", path: contributions_projects_path,
+  active: current_page?(contributions_projects_path)
+) %>