Rewrite "Ban users" functionality

We have some serious problems with current ban system:

• Users don't receive any message about reason and time of ban/unban
• Admins have to remember when user should be unbanned
• Unbanning is fully manual

Work of ban users:
1. "Ban" button

• go to Users -> Ban users menu item;
• select username(-s) from left list, press button "Ban" (pop up window appears)
• combination of value in numeric field and combobox define time for user ban
• if selected value in combobox is "Permamemt" - user(-s) is in permanent ban (despite value in numeric field)
• if button "Save" is pressed - selected users are added in "Banned users" group and receive email with ban reason and expiration date
• after ending ban term user(-s) automatically revoke from group "Banned users" list and receives email notification.

2. "Unban" button

• go to Users -> Ban users menu item;
• in right list select user(-s) for unban.
• press button "Unban "for unban users
• pop up appears with text "Do you want to revoke selected users?", buttons "Ok" and "Cancel". If Admin click "Ok" - users revoke from "Banned users" group and receive email notification

3. "Edit" button

• go to Users -> Ban users menu item;
• in right list select user(-s) for edit;
• pop up window (same as for "Ban" button) appears;
• label "Ban expired" and "Reason" field are filled with current values for selected user(-s)
• Admin can change ban reason and time