Allow locale to be set through the front-end

* Store the last used locale in the session
* Allow home page to be identified in helpers even when containing
  parameters
* Create initializer to explicitly set the available locales, the
  default and the load_path

app/views/layouts/_header.html.erb

@@ -4,6 +4,14 @@
 			<div class="small-12 column">
 				<div class="right">
 					<a href="#"><%= t("layouts.header.external_link_transparency") %></a> | <a href="#"><%= t("layouts.header.external_link_opendata") %></a>
+          |
+          <span id="locale-switcher">
+            [
+              <% available_locales_to_switch.each do |locale| %>
+                <%= link_to(locale, params.merge(locale: locale), id: "locale-link-#{locale}") %>
+              <% end %>
+            ]
+          </span>
 				</div>
 			</div>
 		</div>
@@ -42,4 +50,4 @@
 			</div>
 		</div>
 	<% end %>
-</header>
+</header>