Migrate to rust 2018 edition

Cargo.toml

src/api/core/accounts.rs

src/api/core/ciphers.rs

src/api/core/folders.rs

src/api/core/mod.rs

src/api/core/organizations.rs

src/api/core/two_factor.rs

src/api/icons.rs

src/api/identity.rs

src/api/mod.rs

src/api/notifications.rs

src/api/web.rs

src/auth.rs

src/db/mod.rs

src/db/models/attachment.rs

src/db/models/cipher.rs

src/db/models/collection.rs

src/db/models/device.rs

src/db/models/folder.rs

src/db/models/organization.rs