Fix subject and text of domain invitation

src/registrar/templates/emails/domain_invitation.txt

@@ -1,6 +1,29 @@
-You have been invited to manage the domain {{ domain.name }} on get.gov, 
-the registrar for .gov domain names.
+{% autoescape off %}{# In a text file, we don't want to have HTML entities escaped #}
+Hi $CONFIRM_firstname.
+{{ contact.first_name }}
 
-To accept your invitation, go to <{{ domain_url }}>.
 
-You will need to log in with a Login.gov account using this email address.
+$CONFIRM_name-of-person-who-added-this-person has added you as a manager on {{ domain.name }}.
+{{ application.submitter.first_name }}
+{{ application.submitter.creator }}
+
+YOU NEED A LOGIN.GOV ACCOUNT
+You’ll need a Login.gov account to manage your .gov domain. Login.gov provides a simple and secure process for signing into many government services with one account. If you don’t already have one, follow these steps to create your Login.gov account <https://login.gov/help/get-started/create-your-account/>. 
+
+DOMAIN MANAGEMENT
+As a .gov domain manager you can add or update information about your domain. You’ll also serve as a contact for your .gov domain. Please keep your contact information updated. Learn more about domain management <https://get.gov/help/>.
+
+SOMETHING WRONG?
+If you’re not affiliated with $domain.gov or think you received this message in error, contact the .gov team <https://get.gov/help/#contact-us>. 
+
+
+THANK YOU
+
+.Gov helps the public identify official, trusted information. Thank you for using a .gov domain.
+
+----------------------------------------------------------------
+
+The .gov team
+Contact us: <https://get.gov/contact/>
+Visit <https://get.gov>
+{% endautoescape %}

src/registrar/templates/emails/domain_invitation_subject.txt

@@ -1 +1 @@
-You are invited to manage {{ domain.name }} on get.gov
+You’ve been added to a .gov domain