Resolve #176 "PromotedToSmith is issued even for Smith" (!232)

* fix(#176): last certification does postpone expiry

* test(#176): reveal that last smith certification does not postpone expiry

* fix(#176): clippy

* fix(#176): ExpiresOn is now correctly updated for Pending smiths

* test(#176): show that ExpiresOn is not correctly updated for Pending smiths

* refac(#176): move SmithCertAdded event closer to its storage updates + add docs

* fix(#176): expiration should be postponed only for Pending smiths

* test(#176): reveal that a ceritification on a online smith is bugged

* fix(#176): review: comment

* fix(#176): avoid emitting SmithMembershipAdded twice

* test(#176): reveal bug