embed favicon on backend and make it configurable

config/config.exs

@@ -185,6 +185,7 @@ config :pleroma, :instance,
   short_description: "",
   background_image: "/images/city.jpg",
   instance_thumbnail: "/instance/thumbnail.jpeg",
+  favicon: "/favicon.png",
   limit: 5_000,
   description_limit: 5_000,
   remote_limit: 100_000,

config/description.exs

@@ -987,6 +987,13 @@ config :pleroma, :config_description, [
           "The instance thumbnail can be any image that represents your instance and is used by some apps or services when they display information about your instance.",
         suggestions: ["/instance/thumbnail.jpeg"]
       },
+      %{
+        key: :favicon,
+        type: {:string, :image},
+        description:
+          "Favicon of the instance",
+        suggestions: ["/favicon.png"]
+      },
       %{
         key: :show_reactions,
         type: :boolean,