Implement client get/set display name endpoint

Add ability to unset avatar URL/display name

priv/repo/migrations/20210830160818_create_initial_tables.exs

@@ -6,6 +6,7 @@ defmodule Architex.Repo.Migrations.CreateInitialTables do
       add :localpart, :string, null: false
       add :password_hash, :string, size: 60, null: false
       add :avatar_url, :string, null: true
+      add :displayname, :string, null: true
       timestamps(updated_at: false)
     end