[tor-dev] two new proposals for bridgedb

I separated out the backend database improvements to BridgeDB from the social bridge distributor proposal. The former is finished and up for review and is being submitted to potential funders; the latter is still a draft (though now that some parts of the underlying crypto scheme and the threat model are detailed, early review is always helpful if anyone is super eager to point fingers at any mistakes I've made).

Copies of both proposals are attached, and are also available in the common BridgeDB repo at https://gitweb.torproject.org/bridgedb.git/blob/refs/heads/feature/7520-soci... and https://gitweb.torproject.org/bridgedb.git/blob/refs/heads/feature/7520-soci...