Make nheko usable without a secret storage service

This change adds the ability to run Nheko without a secrets storage
service[1]. The secrets are stored in plaintext under the [secrets]
section of nheko.conf. Since this is a "I know what I'm doing" type
of feature, it's disabled by default and can only be enabled with an
hidden setting `run_without_secure_secrets_service=true`.

Fixes issue #705.

[1]: https://freedesktop.org/wiki/Specifications/secret-storage-spec