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/donations/index.html.erb

@@ -0,0 +1,43 @@
+<%= render HeaderComponent.new(title: "Contributions") %>
+
+<%= render MainWithTabnavComponent.new(tabnav_partial: "shared/tabnav_contributions") do %>
+  <section>
+    <p class="mb-12">
+      Your financial contributions to the development and upkeep of Kosmos
+      software and services.
+    </p>
+    <% if @donations.any? %>
+      <ul class="list-none">
+        <% @donations.each do |donation| %>
+          <li class="mb-8 grid gap-y-2 gap-x-8 grid-cols-2 items-center">
+            <h3 class="mb-0">
+              <%= donation.paid_at.strftime("%B %d, %Y") %>
+            </h3>
+            <p class="row-span-2 font-mono text-right mb-0">
+              <span class="text-xl">
+                <%= number_with_delimiter donation.amount_sats %> sats
+              </span>
+              <br>
+              <span class="text-sm text-gray-500">
+                (~ <%= number_to_currency donation.amount_eur / 100, unit: "" %> EUR)
+              </span>
+            </p>
+            <p class="mb-0">
+              <% if donation.public_name.present? %>
+                Public name: <%= donation.public_name %>
+              <% else %>
+                Anonymous
+              <% end %>
+            </p>
+          </li>
+        <% end %>
+      </ul>
+    <% else %>
+      <p class="text-gray-500">
+        The donation process is not automated yet. Please
+        <a href="https://wiki.kosmos.org/Main_Page#Community_.2F_Getting_in_touch_.2F_Getting_involved" class="ks-text-link" target="_blank">contact us</a>
+        if you'd like to contribute this way right now.
+      </p>
+    <% end %>
+  </section>
+<% end %>

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 %>