Ability to opt in to groups via the members interface

[tbar0970]

It would be handy if people who have logged into the members interface were able to opt themselves into certain groups. This would be useful for several things:

1. Roster recruitment: Get people to go to the Jethro members area and sign up for "Bible reading volunteers 2019" for example.
2. Member directory: People who've joined the church and registered and accesed the members area can opt into the "church directory" group.

You would only want to make some groups available for self-opt-in.

In some cases (like number 2 above) you'd need some sort of explanatory text: "By joining this group you are making your details visible to the other group members. You also agree not to mis-use the contact information that is shared with you".

In some cases (like number 2 above) you would want to restrict who could opt themselves in. For example only people with status core or crowd, only adults, only members of 9am congregation. This could potentially be complex (it's the kind of filtering done by a report). But in practice basic fields like status, age bracket and congregation are likely to be the only filters needed.