Add migration and schema for accounts

Add account name availability endpoint

lib/matrix_server/account.ex

@@ -0,0 +1,51 @@
+defmodule MatrixServer.Account do
+  use Ecto.Schema
+  import Ecto.{Changeset, Query}
+  alias MatrixServer.{Repo, Account}
+
+  @max_mxid_length 255
+  @localpart_regex ~r/^([a-z0-9\._=\/])+$/
+
+  @primary_key {:localpart, :string, []}
+  schema "accounts" do
+    field :password_hash, :string, redact: true
+
+    timestamps(updated_at: false)
+  end
+
+  def available?(localpart) when is_binary(localpart) do
+    if Regex.match?(@localpart_regex, localpart) and
+         String.length(localpart) <= localpart_length() do
+      if Repo.one!(
+           Account
+           |> where([a], a.localpart == ^localpart)
+           |> select([a], count(a))
+         ) == 0 do
+        :ok
+      else
+        {:error, :user_in_use}
+      end
+    else
+      {:error, :invalid_username}
+    end
+  end
+
+  def changeset(%Account{} = account, attrs) do
+    account
+    |> cast(attrs, [:localpart, :password_hash])
+    |> validate_required([:localpart, :password_hash])
+    |> validate_length(:password_hash, max: 60)
+    |> validate_format(:localpart, @localpart_regex)
+    |> validate_length(:localpart, max: localpart_length())
+    |> unique_constraint(:localpart, name: :accounts_pkey)
+  end
+
+  defp localpart_length do
+    # Subtract the "@" and ":" in the MXID.
+    @max_mxid_length - 2 - String.length(server_name())
+  end
+
+  defp server_name do
+    Application.get_env(:matrix_server, :server_name)
+  end
+end