New Admin Email subject line should be filterable

[MadtownLems]

When a user attempts to update the Administration Email Address (Settings->Reading), they receive an email with a link they must click to confirm the change.

The body of the email is filterable (new_admin_email_content), but sadly the subject is not.

I'm proposing adding a new filter, such as new_admin_email_subject, that lets users customize this message.

For reference, I was looking to make it clearer that there is still action needed (to confirm the change), because I've seen LOTS of situations on our networks where these email updates are started - but never confirmed - likely due to the subject line sounding way more like a confirmation than something that requires action.

[alexanderkoledov]

Filter "new_admin_email_subject" added