Display customized mascot in web UI and fix admin form for it (#8964)

Follow-up to #8766

app/javascript/mastodon/features/compose/index.js

@@ -13,6 +13,7 @@ import spring from 'react-motion/lib/spring';
 import SearchResultsContainer from './containers/search_results_container';
 import { changeComposing } from '../../actions/compose';
 import elephantUIPlane from '../../../images/elephant_ui_plane.svg';
+import { mascot } from '../../initial_state';
 
 const messages = defineMessages({
   start: { id: 'getting_started.heading', defaultMessage: 'Getting started' },
@@ -107,7 +108,7 @@ class Compose extends React.PureComponent {
             <ComposeFormContainer />
             {multiColumn && (
               <div className='drawer__inner__mastodon'>
-                <img alt='' draggable='false' src={elephantUIPlane} />
+                <img alt='' draggable='false' src={mascot || elephantUIPlane} />
               </div>
             )}
           </div>}

app/javascript/mastodon/initial_state.js

@@ -14,5 +14,6 @@ export const me = getMeta('me');
 export const searchEnabled = getMeta('search_enabled');
 export const invitesEnabled = getMeta('invites_enabled');
 export const version = getMeta('version');
+export const mascot = getMeta('mascot');
 
 export default initialState;

app/serializers/initial_state_serializer.rb

@@ -16,6 +16,7 @@ class InitialStateSerializer < ActiveModel::Serializer
       search_enabled: Chewy.enabled?,
       version: Mastodon::Version.to_s,
       invites_enabled: Setting.min_invite_role == 'user',
+      mascot: instance_presenter.mascot&.file&.url,
     }
 
     if object.current_account
@@ -56,4 +57,10 @@ class InitialStateSerializer < ActiveModel::Serializer
   def media_attachments
     { accept_content_types: MediaAttachment::IMAGE_FILE_EXTENSIONS + MediaAttachment::VIDEO_FILE_EXTENSIONS + MediaAttachment::IMAGE_MIME_TYPES + MediaAttachment::VIDEO_MIME_TYPES }
   end
+
+  private
+
+  def instance_presenter
+    @instance_presenter ||= InstancePresenter.new
+  end
 end

app/views/admin/settings/edit.html.haml

@@ -26,6 +26,8 @@
       = f.input :thumbnail, as: :file, wrapper: :with_block_label, label: t('admin.settings.thumbnail.title'), hint: t('admin.settings.thumbnail.desc_html')
     .fields-row__column.fields-row__column-6.fields-group
       = f.input :hero, as: :file, wrapper: :with_block_label, label: t('admin.settings.hero.title'), hint: t('admin.settings.hero.desc_html')
+
+  .fields-row
     .fields-row__column.fields-row__column-6.fields-group
       = f.input :mascot, as: :file, wrapper: :with_block_label, label: t('admin.settings.mascot.title'), hint: t('admin.settings.mascot.desc_html')