Account Key must not be saved to keychain

[dkocher]

Account Key must not be saved to keychain as per https://github.com/shift7-ch/katta-arc42/pull/1/files#diff-b335630551682c19a781afebcf4d07bf978fb1f8ac04c6bf87428ed5106870f5R333-R336. The account key can be changed in the web UI and therefore this can lead to inconsistencies.

Originally posted by @chenkins in #127 (comment)

https://github.com/shift7-ch/katta-docs/blob/44819f4e44a61991261d7ecac1bb40c1541603b7/ARCHIECTURE.md#decision-35-write-setup-code-to-keychain-upon-first-login-or-setup-device-with-setup-code

[dkocher]

See also #35.