Implementation of cert stock (#383)

res/ui/certification.ui

src/sakia/core/registry/identity.py

src/sakia/gui/certification.py

src/sakia/tests/mocks/bma/init_new_community.py

src/sakia/tests/mocks/bma/new_blockchain.py

src/sakia/tests/mocks/bma/nice_blockchain.py

src/sakia/tests/unit/core/test_identity.py