Add fallback submission email address for database seed

Forms can't be made live without a submission email address, so if the
`EMAIL` environment is not set or an email address can't be retrieved
with git then initializing the database will fail.

This commit uses an example email address as a fallback so that the
database seed can always be applied. We also update the README to
document the behaviour of the submission emails for test forms.

Author: lfdebrux

README.md

@@ -127,6 +127,8 @@ To run this project, your database will need to have a user in it. The `bin/setu
 ./bin/rails db:seed
 ```
 
+The database seed will also create a number of test forms, by default the submissions for these forms will be sent by email to the email address that you have configured in Git. If you want to override this, or are running this app in a container where git or its configuration is not present, then you can use set the `EMAIL` environment variable. If neither `EMAIL` or the git email address are set, the database seed will fallback to a generic test email address.
+
 ## Changing configuration
 
 ### Changing settings

db/seeds.rb

@@ -134,7 +134,7 @@
 
   Membership.create! user: default_user, group: end_to_end_group, added_by: default_user, role: :group_admin
 
-  submission_email = ENV["EMAIL"] || `git config --get user.email`.strip
+  submission_email = ENV["EMAIL"].presence || `git config --get user.email`.strip.presence || "example@example.com"
 
   all_question_types_form = Form.create!(
     name: "All question types form",