Added a username reminder for admins

[lwcorp]

Description

Although password reminder insecurely mentions the username, it does it without using the word "username", but either way there should be an independent username reminder without the possibility to accidentally modify the database and change the password.

As a side fix, the e-mail field was defined as text rather than email.

All of this was done by adding support in:

1. Frontend
2. Backend
3. E-mail itself

I didn't duplicate public_html/lists/admin/tests/subscriber_password.php since it's meant for end-users while this task is for admins.

Note this requires the following new translations:

For public_html/lists/admin/tests/admin/index.php:

1. Failed sending a username reminder

For public_html/lists/admin/tests/admin/lib.php:

1. You have requested a username reminder for phpList.
2. Your username is:
3. Username reminder
4. A username reminder has been sent to the corresponding email address.
5. Error sending user reminder

For public_html/lists/admin/tests/admin/login.php:

1. Forgot username
2. Enter your email address
3. Send username

Related Issue

N/A

Screenshots (if appropriate):

[lwcorp]

Note the checks failed due to #1123.