Transitive peer discovery

[LGro]

When we have the concept of circles (#12), we might allow users to opt-into their peers sharing some part of their contact data with others in the same circle for peer discovery. Flesh out this idea here, including privacy risk scenarios.

[LGro]

I feel like "introducing folks to each other" seems like a useful paradigm that users have a mental representation of (as opposed to the "out of control" someone has a circle and my details end up there straight away. One user could create DHT records to introduce two of their contacts to each other; and if they provide them each others' pubkeys they get the trust and also the secret channel. It might also be useful for users to define a peer introduction profile subset of information that is shared.

[LGro]

The smaller introduction profile needs to at least include one contact detail to uniquely identify a contact (like mail, phone, social, website).

[LGro]

We could use the same to introduce new people to others for a reason like A is visiting a place and asking B via Cogulate to recommend contacts in that place if they have any, which they can do given that their contacts have agreed to being referred and provided a peer introduction profile. We might want to handle this in a separate issue.

[LGro]

There could be a predefined circle "introducers" that can be used to configure the introduction profile and contacts that are allowed to make introductions. It would need to be hard coded, the label localised, and protected from deletion. I'm thinking about enabling this functionality with as little additional UI elements / paradigms as possible to avoid overcomplicating the UI. We probably still need one new screen or pop over where one can propose introductions. Maybe from the updates page, when we "notify" users to propose introductions, or on the details page.