Hook up user validations (#58)

While working on updating a user through their profile page, we found
that the first and last names were lacking presence validations.

app/models/user.rb

@@ -3,6 +3,8 @@
 class User < ApplicationRecord
   include Clearance::User
 
+  validates :first_name, :last_name, presence: true
+
   has_many :memberships
   has_many :commitments, through: :memberships
   has_many :groups, through: :memberships

spec/models/user_spec.rb

@@ -0,0 +1,10 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+RSpec.describe User do
+  describe "validations" do
+    it { should validate_presence_of(:first_name) }
+    it { should validate_presence_of(:last_name) }
+  end
+end