Fix welcome text for better grammar in themed instances

lib/public/Mail/IEMailTemplate.php

settings/Mailer/NewUserMailHelper.php

tests/Settings/Mailer/NewUserMailHelperTest.php

...data/emails/new-account-email-custom-text-alternative.txt

tests/data/emails/new-account-email-custom.html

tests/data/emails/new-account-email-custom.txt

tests/data/emails/new-account-email-single-button.html

tests/data/emails/new-account-email-single-button.txt

tests/data/emails/new-account-email.html

tests/data/emails/new-account-email.txt

tests/lib/Mail/EMailTemplateTest.php