notification email preferences for completed membership requests not on settings page

[tharsheblows]

There's no way to opt out of the emails sent when group membership requests have been accepted or denied.

[tharsheblows]

adds in field to email settings to opt out of group membership completed emails

[DJPaul]

Nice, thanks!

[djpaul]

In 10718:

Emails: add option to notifications settings to opt-out of emails sent when group membership requests are accepted/rejected.

Fixes #7029

Props tharsheblows