Can't link several account of the same social network to my keybase account

Currently, there is no (obvious?) way to link several accounts from the same social network provider with a given Keybase account.

A typical use case would be proving the owner identity of multiple Twitter accounts (and as a side effect, proving them to be "sibling" accounts).

Another possible use case would be allowing an overlap period between a transition from one account to another one.

Related to:
* #178 (several use cases mentioned in the discussion)
* #2476 (specific to GitHub)

Start by reading the related discussions in #178 and #2476, especially the GitHub-specific case, to understand the existing account-linking constraints. Define what support for multiple accounts from one social network and an account transition period would mean, then verify that the resulting behavior covers the use cases described here.

Written by the indexing model from the issue text.