Add a mode where every new user is muted and a button for admins to unmute

[kommandantredundant]

In small and/or private groups, this can be useful, when the group is somehow public but shall not be joined by random users (eg. group is in grouplists of games).
The option could look like the captcha message, but instead provide a button for admins/elevates only (there could also be a option where every user can unmute new members). The admin presses the button, the member gets unmuted and a message gets logged.

[OlgabrezelPrivate]

In combination with #58:

Let's add a new setting to add an "Unmute" button to the welcome message.

If both captcha and unmute button are turned on, both are sent and using either of them unmutes the user.

If only unmute button is turned on, it will work like the original suggestion of this issue.