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/contributions/projects/index.html.erb

@@ -0,0 +1,16 @@
+<%= render HeaderComponent.new(title: "Contributions") %>
+
+<%= render MainWithTabnavComponent.new(tabnav_partial: "shared/tabnav_contributions") do %>
+  <section>
+    <p class="mb-8">
+      Project contributions are how we develop and run all Kosmos software and
+      services. Everything we create and provide is free and open-source
+      software, even the page you're looking at right now!
+    </p>
+    <p>
+      Soon you will find a summary of your contributions here. Until then,
+      please refer to the
+      <a href="https://kredits.kosmos.org/" class="ks-text-link" target="_blank">Kredits dashboard</a>.
+    </p>
+  </section>
+<% end %>