Add alt attribute to map images

We were using it on the map displayed on the sidebar, but we weren't
using it in the other places were the map is rendered.

app/views/budgets/groups/show.html.erb

@@ -23,6 +23,6 @@
   </div>
 
   <div class="medium-5 column show-for-medium text-center">
-    <%= image_tag(image_path_for("map.jpg")) %>
+    <%= image_tag(image_path_for("map.jpg"), alt: t("shared.tags_cloud.districts_list")) %>
   </div>
 </div>

app/views/proposals/map.html.erb

@@ -13,7 +13,7 @@
       </div>
 
       <div class="show-for-medium medium-9 column text-center">
-        <%= image_tag(image_path_for("map.jpg"), usemap: "#map") %>
+        <%= image_tag(image_path_for("map.jpg"), alt: t("shared.tags_cloud.districts_list"), usemap: "#map") %>
       </div>
 
       <map name="map" id="html_map">

spec/system/admin/site_customization/images_spec.rb

@@ -45,19 +45,19 @@ describe "Admin custom images", :admin do
     visit proposals_path
 
     within("#map") do
-      expect(page).to have_css("img[src*='custom_map.jpg']")
+      expect(page).to have_css("img[src*='custom_map.jpg'][alt='Districts list']")
     end
 
     visit map_proposals_path
 
     within(".show-for-medium") do
-      expect(page).to have_css("img[src*='custom_map.jpg']")
+      expect(page).to have_css("img[src*='custom_map.jpg'][alt='Districts list']")
     end
 
     visit budget_group_path(budget, group)
 
     within(".show-for-medium") do
-      expect(page).to have_css("img[src*='custom_map.jpg']")
+      expect(page).to have_css("img[src*='custom_map.jpg'][alt='Districts list']")
     end
   end