Add customizable thumbnails for audio and video attachments (#14145)

- Change audio files to not be stripped of metadata
- Automatically extract cover art from audio if it exists
- Add `thumbnail` parameter to `POST /api/v1/media`, `POST /api/v2/media` and `PUT /api/v1/media/:id`
- Add `icon` to represent it in attachments in ActivityPub
- Fix `preview_url` containing URL of missing missing image when there is no thumbnail instead of null
- Fix duration of audio not being displayed on public pages until the file is loaded

db/migrate/20200627125810_add_thumbnail_columns_to_media_attachments.rb

@@ -0,0 +1,11 @@
+class AddThumbnailColumnsToMediaAttachments < ActiveRecord::Migration[5.2]
+  def up
+    add_attachment :media_attachments, :thumbnail
+    add_column :media_attachments, :thumbnail_remote_url, :string
+  end
+
+  def down
+    remove_attachment :media_attachments, :thumbnail
+    remove_column :media_attachments, :thumbnail_remote_url
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2020_06_20_164023) do
+ActiveRecord::Schema.define(version: 2020_06_27_125810) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -489,6 +489,11 @@ ActiveRecord::Schema.define(version: 2020_06_20_164023) do
     t.string "blurhash"
     t.integer "processing"
     t.integer "file_storage_schema_version"
+    t.string "thumbnail_file_name"
+    t.string "thumbnail_content_type"
+    t.integer "thumbnail_file_size"
+    t.datetime "thumbnail_updated_at"
+    t.string "thumbnail_remote_url"
     t.index ["account_id"], name: "index_media_attachments_on_account_id"
     t.index ["scheduled_status_id"], name: "index_media_attachments_on_scheduled_status_id"
     t.index ["shortcode"], name: "index_media_attachments_on_shortcode", unique: true