Add custom closed registrations message to landing page when set (#10347)

app/javascript/styles/mastodon/forms.scss

@@ -475,6 +475,42 @@ code {
       }
     }
   }
+
+  &__overlay-area {
+    position: relative;
+
+    &__overlay {
+      position: absolute;
+      top: 0;
+      left: 0;
+      width: 100%;
+      height: 100%;
+      display: flex;
+      justify-content: center;
+      align-items: center;
+      background: rgba($ui-base-color, 0.65);
+      backdrop-filter: blur(2px);
+      border-radius: 4px;
+
+      &__content {
+        text-align: center;
+
+        &.rich-formatting {
+          &,
+          p {
+            color: $primary-text-color;
+          }
+        }
+      }
+    }
+  }
+}
+
+.block-icon {
+  display: block;
+  margin: 0 auto;
+  margin-bottom: 10px;
+  font-size: 24px;
 }
 
 .flash-message {

app/presenters/instance_presenter.rb

@@ -8,6 +8,7 @@ class InstancePresenter
     :site_description,
     :site_extended_description,
     :site_terms,
+    :closed_registrations_message,
     to: Setting
   )
 

app/views/about/_registration.html.haml

@@ -1,4 +1,5 @@
 = simple_form_for(new_user, url: user_registration_path) do |f|
+  .simple_form__overlay-area
     %p.lead= t('about.federation_hint_html', instance: content_tag(:strong, site_hostname))
 
     .fields-group
@@ -14,3 +15,9 @@
 
     .actions
       = f.button :button, sign_up_message, type: :submit, class: 'button button-primary', disabled: closed_registrations?
+
+    - if closed_registrations? && @instance_presenter.closed_registrations_message.present?
+      .simple_form__overlay-area__overlay
+        .simple_form__overlay-area__overlay__content.rich-formatting
+          .block-icon= fa_icon 'warning'
+          = @instance_presenter.closed_registrations_message.html_safe