Minor textual updates in the "Forgot Username" email

This is the email text in the default template for the "Forgot Username" email message:

Hello $firstname $lastname,

You have requested the username for your email: $email.

Your username is: $username

If you think it was sent incorrectly, please contact one of the administrators at: $admincontact

$deploymenttitle Administrator

Here is an example of an email message sent when there were 2 usernames for a single email address:

Hello Admin Administrator,

You have requested the username for your email: smaddox@atlassian.com.

Your username is: admin, arthur

If you think it was sent incorrectly, please contact one of the administrators at: smaddox@atlassian.com

Crowd Administrator

Please change the following:

• Change "email" to "email address" in this line:

  You have requested the username for your email: $email
  

• Change "username is" to "username(s) are" in this line:

  Your username is:
  

  It would be even better if we could generate the correct grammar: if there is only one username, then send "username is" and if there are two or more usernames, then send "usernames are". Is that possible, given that this is the default template?

• Change "it" to "this email" in this line:

  If you think it was sent incorrectly,...