Option to disable receiving private messages per user

Problem/Motivation

Hi,

Currently, in the user settings there are:

Receive notification of private messages (checkbox)

Send notifications of new messages in a thread

• For every private message (radio button)
• Only when not viewing the thread (radio button)

Amount of time after leaving a thread that the system starts sending notifications of new messages
10 minutes (drop down)

---new feature request ----

There should be option where user could completely stop receiving private messages.
For example "disable private messages" or "stop receiving private messages"

If user X selects this option in his own user page, he won't appear anymore in the /private-message/create
"To:" field. So nobody can send to him anymore new messages. He can still message in the existing threads.

Why this should be done?

1. Now any user can send private message to admin (typing admin to the To: field in the /private-message/create page)
2. Admin username is revealed (for example I am using not admin, but adminz and that is revealed now in the To: field)
3. Not all user's want's to receive messages, there has to be "stop receiving private messages" for those who do not want to use this feature.
4. Privacy, some users would like to hide their existence in the site. Now typing anyting in the To: field shows all user names

Thanks for the great module!

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Add "disable private messages" checkbox in the user settings.

API changes

Data model changes