Return character limit in API and initial state.

app/serializers/initial_state_serializer.rb

@@ -2,10 +2,15 @@
 
 class InitialStateSerializer < ActiveModel::Serializer
   attributes :meta, :compose, :accounts,
-             :media_attachments, :settings, :push_subscription
+             :media_attachments, :settings, :push_subscription,
+             :max_chars
 
   has_many :custom_emojis, serializer: REST::CustomEmojiSerializer
 
+  def max_chars
+    StatusLengthValidator::MAX_CHARS
+  end
+
   def custom_emojis
     CustomEmoji.local.where(disabled: false)
   end