constants: add as_public constant and use it everywhere

lib/pleroma/web/ostatus/activity_representer.ex

@@ -9,6 +9,7 @@ defmodule Pleroma.Web.OStatus.ActivityRepresenter do
   alias Pleroma.Web.OStatus.UserRepresenter
 
   require Logger
+  require Pleroma.Constants
 
   defp get_href(id) do
     with %Object{data: %{"external_url" => external_url}} <- Object.get_cached_by_ap_id(id) do
@@ -34,7 +35,7 @@ defmodule Pleroma.Web.OStatus.ActivityRepresenter do
     Enum.map(to, fn id ->
       cond do
         # Special handling for the AP/Ostatus public collections
-        "https://www.w3.org/ns/activitystreams#Public" == id ->
+        Pleroma.Constants.as_public() == id ->
           {:link,
            [
              rel: "mentioned",

lib/pleroma/web/ostatus/handlers/note_handler.ex

@@ -4,6 +4,7 @@
 
 defmodule Pleroma.Web.OStatus.NoteHandler do
   require Logger
+  require Pleroma.Constants
 
   alias Pleroma.Activity
   alias Pleroma.Object
@@ -49,7 +50,7 @@ defmodule Pleroma.Web.OStatus.NoteHandler do
   def get_collection_mentions(entry) do
     transmogrify = fn
       "http://activityschema.org/collection/public" ->
-        "https://www.w3.org/ns/activitystreams#Public"
+        Pleroma.Constants.as_public()
 
       group ->
         group
@@ -126,7 +127,7 @@ defmodule Pleroma.Web.OStatus.NoteHandler do
          to <- make_to_list(actor, mentions),
          date <- XML.string_from_xpath("//published", entry),
          unlisted <- XML.string_from_xpath("//mastodon:scope", entry) == "unlisted",
-         cc <- if(unlisted, do: ["https://www.w3.org/ns/activitystreams#Public"], else: []),
+         cc <- if(unlisted, do: [Pleroma.Constants.as_public()], else: []),
          note <-
            CommonAPI.Utils.make_note_data(
              actor.ap_id,