Use just the banner title as the banner link text

We were using an <a> tag wrapping the whole content of the banner in
order to make the whole banner clickable. However, that made the text of
the link less concise, affecting people using screen readers. So,
instead, we're using the `card` mixin, which we introduced in commit
f285dfcba.

We're making this change now because the HTML5 Sanitizer that we're
about to enable in the next commit was handling the whitespace inside
the banner differently, causing one test to fail, and we didn't find a
different way to fix it.

app/assets/stylesheets/shared/banner.scss

@@ -1,5 +1,7 @@
 .banner {
   @include full-width-background;
+  @include card;
+  padding: 0 rem-calc(16);
 
   .debates-list &::before,
   .proposals-list &::before,
@@ -7,18 +9,21 @@
     content: none;
   }
 
+  &:hover {
+    text-decoration: underline;
+  }
+
   a {
-    display: block;
-
-    > * {
-      padding: 0 rem-calc(16);
-
-      &:empty {
-        display: none;
-      }
+    &,
+    &:hover {
+      color: inherit;
     }
   }
 
+  > :empty {
+    display: none;
+  }
+
   + .budget-header,
   + .budgets-index > .budget-header,
   + .jumbo {

app/components/shared/banner_component.html.erb

@@ -1,3 +1,3 @@
 <div class="banner" style="background-color:<%= banner.background_color %>;">
-  <%= sanitize link, attributes: %w[href style] %>
+  <%= sanitize banner_content, attributes: %w[href style] %>
 </div>

app/components/shared/banner_component.rb

@@ -19,10 +19,8 @@ class Shared::BannerComponent < ApplicationComponent
 
   private
 
-    def link
-      link_to banner.target_url do
-        tag.h2(banner.title, style: "color:#{banner.font_color}") +
-          tag.h3(banner.description, style: "color:#{banner.font_color}")
-      end
+    def banner_content
+      tag.h2(link_to(banner.title, banner.target_url), style: "color:#{banner.font_color}") +
+        tag.h3(banner.description, style: "color:#{banner.font_color}")
     end
 end

spec/system/admin/banners_spec.rb

@@ -86,7 +86,7 @@ describe "Admin banners magement", :admin do
     visit proposals_path
 
     expect(page).to have_content "Such banner"
-    expect(page).to have_link "Such banner many text wow link", href: "https://www.url.com"
+    expect(page).to have_link "Such banner", href: "https://www.url.com"
   end
 
   scenario "Publish a banner with a translation different than the current locale" do