Limit emoji stealer to alphanum, dash, or underscore characters

As suggested in b387f4a1c1, only steal
emoji with alphanumerc, dash, or underscore characters.

Also consolidate all validation logic into a single function.

lib/pleroma/web/activity_pub/mrf/steal_emoji_policy.ex

@@ -20,6 +20,17 @@ defmodule Pleroma.Web.ActivityPub.MRF.StealEmojiPolicy do
     String.match?(shortcode, pattern)
   end
 
+  defp reject_emoji?({shortcode, _url}, installed_emoji) do
+    invalid_shortcode? = !String.match?(shortcode, ~r/^[a-zA-Z0-9_-]+$/)
+
+    rejected_shortcode? =
+      [:mrf_steal_emoji, :rejected_shortcodes]
+      |> Config.get([])
+      |> Enum.any?(fn pattern -> shortcode_matches?(shortcode, pattern) end)
+
+    shortcode in installed_emoji or invalid_shortcode? or rejected_shortcode?
+  end
+
   defp steal_emoji({shortcode, url}, emoji_dir_path) do
     url = Pleroma.Web.MediaProxy.url(url)
 
@@ -76,18 +87,7 @@ defmodule Pleroma.Web.ActivityPub.MRF.StealEmojiPolicy do
 
       new_emojis =
         foreign_emojis
-        |> Enum.reject(fn {shortcode, _url} -> shortcode in installed_emoji end)
-        |> Enum.reject(fn {shortcode, _url} ->
-          String.contains?(shortcode, ["/", "\\", ".", ":"])
-        end)
-        |> Enum.filter(fn {shortcode, _url} ->
-          reject_emoji? =
-            [:mrf_steal_emoji, :rejected_shortcodes]
-            |> Config.get([])
-            |> Enum.find(false, fn pattern -> shortcode_matches?(shortcode, pattern) end)
-
-          !reject_emoji?
-        end)
+        |> Enum.reject(&reject_emoji?(&1, installed_emoji))
         |> Enum.map(&steal_emoji(&1, emoji_dir_path))
         |> Enum.filter(& &1)