Add regex validation to user email

Fixes in delayed job to deliver mails:
```
501 Recipient syntax error

/usr/local/lib/ruby/2.5.0/net/smtp.rb:969:in `check_response'
/usr/local/lib/ruby/2.5.0/net/smtp.rb:937:in `getok'
/usr/local/lib/ruby/2.5.0/net/smtp.rb:865:in `rcptto'
/usr/local/lib/ruby/2.5.0/net/smtp.rb:846:in `block in rcptto_list'
```

app/models/user.rb

@@ -28,7 +28,7 @@ class User < ApplicationRecord
   after_validation :set_unconfirmed_email, if: :email_changed?, on: :update
   before_create :generate_api_key, :generate_confirmation_token
 
-  validates :email, length: { maximum: 254 }
+  validates :email, length: { maximum: 254 }, format: { with: URI::MailTo::EMAIL_REGEXP }
 
   validates :handle, uniqueness: true, allow_nil: true
   validates :handle, format: {

test/unit/helpers/dynamic_errors_helper_test.rb

@@ -4,8 +4,9 @@ class DynamicErrorsHelperTest < ActionView::TestCase
   test "returns a div with the errors if the object is invalid" do
     user = build(:user, email: nil)
     user.valid?
-    expected_dom = %(<div class="errorExplanation" id="errorExplanation"><h2>2 errors prohibited this user from being saved</h2><p>There were \
-problems with the following fields:</p><ul><li>Email address is not a valid email</li><li>Email address can't be blank</li></ul></div>)
+    expected_dom = %(<div class="errorExplanation" id="errorExplanation"><h2>3 errors prohibited this user from being saved</h2><p>There were \
+problems with the following fields:</p><ul><li>Email address is not a valid email</li><li>Email address can't be blank</li><li>Email address is \
+invalid</li></ul></div>)
 
     assert_dom_equal expected_dom, error_messages_for(user)
   end

test/unit/user_test.rb

@@ -60,6 +60,17 @@ def assert_resetting_email_changes(attr_name)
         refute user.valid?
         assert_contains user.errors[:email], "is too long (maximum is 254 characters)"
       end
+
+      should "be valid when it matches URI mail email regex" do
+        user = build(:user, email: "mail@example.com")
+        assert user.valid?
+      end
+
+      should "be invalid when it doesn't match URI mail email regex" do
+        user = build(:user, email: "random[a..z]mdhlwqui@163.com")
+        refute user.valid?
+        assert_contains user.errors[:email], "is invalid"
+      end
     end
 
     context "twitter_username" do