Allow groups to set the default new user preferences

[davidearl]

In particular, to set email notifications on by default. IMO, less deeply involved people may well sign up and then never visit again because they are never prompted to and they forget about it, forget how to do it, etc.

[mvl22]

Increasingly clear to me that this would solve a lot of issues.

When a user is being invited onto the system as a result of a group invitation, I think the group's norms should basically be cloned. This means, in the case of say, Camcycle, a group which has historically been very e-mail bound, having the e-mail flag ticked by default, is sensible.

(If a user is then invited by a second group, their preferences would stay - they can adjust things themselves from that point.)

[mvl22]

Suggestion (on a different topic) at #425:

"I really think the area should be defaulted to the group area (very occasionally more than one if they are in more than one group). Marking out areas is one thing people consistently miss the steps for (even though they are really prominent) and fail to complete. Again this is much more fail safe - they can always redefine them, but if they don't know how to get them in the first place they are in limbo"

Exactly the same issue as default preferences apply, so this should probably be implemented together.