Loosen the validation regex of emoji reaction a bit

We couldn't previously react with an emoji whose shortcode contained
dots, spaces or any non-ascii characters.

The only rule we're now enforcing is that it might contain only one `@`
character (used to distinguish external emoji) and must be wrapped with
colon symbols.

lib/pleroma/web/activity_pub/object_validators/emoji_react_validator.ex

@@ -13,7 +13,7 @@ defmodule Pleroma.Web.ActivityPub.ObjectValidators.EmojiReactValidator do
   import Pleroma.Web.ActivityPub.ObjectValidators.CommonValidations
 
   @primary_key false
-  @emoji_regex ~r/:[A-Za-z0-9_-]+(@.+)?:/
+  @emoji_regex ~r/^:[^@]+(@[^@]+)?:$/
 
   embedded_schema do
     quote do