Don't steal emoji who's shortcodes have dots or colons in their name

Mastodon at the very least seems to prevent the creation of emoji with
dots in their name (and refuses to accept them in federation). It feels
like being cautious in what we accept is reasonable here.

Colons are the emoji separator and so obviously should be blocked.

Perhaps instead of filtering out things like this we should just
do a regex match on `[a-zA-Z0-9_-]`? But that's plausibly a decision
for another day

    Perhaps we should also have a centralised "is this a valid emoji shortcode?"
    function

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

@@ -77,7 +77,7 @@ 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.reject(fn {shortcode, _url} -> String.contains?(shortcode, ["/", "\\", ".", ":"]) end)
         |> Enum.filter(fn {shortcode, _url} ->
           reject_emoji? =
             [:mrf_steal_emoji, :rejected_shortcodes]