Fix the confusingly named and inverted logic of "no_attachment_links"

The setting is now simply "attachment_links" and the boolean value does
what you expect. A double negative is never possible and describing the
functionality is no longer a philospher's worst nightmare.

config/config.exs

@@ -241,7 +241,7 @@ config :pleroma, :instance,
   mrf_transparency_exclusions: [],
   autofollowed_nicknames: [],
   max_pinned_statuses: 1,
-  no_attachment_links: true,
+  attachment_links: false,
   welcome_user_nickname: nil,
   welcome_message: nil,
   max_report_comment_size: 1000,

config/description.exs

@@ -796,10 +796,10 @@ config :pleroma, :config_description, [
         ]
       },
       %{
-        key: :no_attachment_links,
+        key: :attachment_links,
         type: :boolean,
         description:
-          "Set to `true` to disable automatically adding attachment link text to statuses"
+          "Enable to automatically add attachment link text to statuses"
       },
       %{
         key: :welcome_message,