Implement client get/set display name endpoint

Add ability to unset avatar URL/display name

lib/architex/schema/account.ex

@@ -16,7 +16,8 @@ defmodule Architex.Account do
   schema "accounts" do
     field :localpart, :string
     field :password_hash, :string, redact: true
-    field :avatar_url
+    field :avatar_url, :string
+    field :displayname, :string
     has_many :devices, Device
 
     many_to_many :joined_rooms, Room,