This changes the way environment variables are read into Vikunja's config.
With this change, Vikunja loads the config from all env variables into the config store explicitly, after all config files have been processed. The breaking change here is that values from env variables now may override values from a config file when both are specified.
This allows specifying openid providers using only environment values. Previously this still required a config file to work, because viper wouldn't know about these values otherwise.
Resolves https://community.vikunja.io/t/configure-openid-via-environment/628/16
5c02527d2d