Initial support for soft deletes

...s/mysql/2020-04-09-235005_add_cipher_delete_date/down.sql

...ons/mysql/2020-04-09-235005_add_cipher_delete_date/up.sql

...tgresql/2020-04-09-235005_add_cipher_delete_date/down.sql

...ostgresql/2020-04-09-235005_add_cipher_delete_date/up.sql

.../sqlite/2020-04-09-235005_add_cipher_delete_date/down.sql

...ns/sqlite/2020-04-09-235005_add_cipher_delete_date/up.sql

src/api/core/ciphers.rs

src/db/models/cipher.rs

src/db/schemas/mysql/schema.rs

src/db/schemas/postgresql/schema.rs

src/db/schemas/sqlite/schema.rs