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.

===

Taken from akkoma#703 with cosmetic tweaks

This matches our existing validation logic from Pleroma.Emoji,
and apart from excluding the dot also POSIX’s Portable Filename
Character Set making it always safe for use in filenames.

Mastodon is even stricter also disallowing U+002D HYPEN-MINUS
and requiring at least two characters.

Given both we and Mastodon reject shortcodes excluded
by this anyway, this doesn’t seem like a loss.

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

@@ -20,6 +20,19 @@ defmodule Pleroma.Web.ActivityPub.MRF.StealEmojiPolicy do
     String.match?(shortcode, pattern)
   end
 
+  defp reject_emoji?({shortcode, _url}, installed_emoji) do
+    valid_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)
+
+    emoji_installed? = Enum.member?(installed_emoji, shortcode)
+
+    !valid_shortcode? or rejected_shortcode? or emoji_installed?
+  end
+
   defp steal_emoji({shortcode, url}, emoji_dir_path) do
     url = Pleroma.Web.MediaProxy.url(url)
 
@@ -76,18 +89,7 @@ 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)