Manage secrets outside of server_config.json

[barnabasdomozi]

In this PR, I added an option to store secrets outside of server_config.json.

To use this feature, we can replace sensitive data with $SECRET:NAME_OF_SECRET$, then NAME_OF_SECRET will be read from a separate secrets.json file when we initially load server_config.json.
This change is also backward compatible (e.g., secrets can still be stored in server_config.json).

See the updated documentation in docs/web/server_config.md for more info.

[bruntib]

Please, fix the lint test, otherwise looks good to me.

[Discookie]

Can these secrets only be a string, or can they be arbitrary JSON data?
It seems that currently the secrets.json can store arbitrary data, while the environment variable cannot.

Not sure if loading arbitrary data is intentional, but it would be useful to offload eg. the entire dictionary-based auth dictionary into the secrets, instead of each individual password.
If it's not intentional, it should be checked against, when loading the secrets.json.

[Discookie]

(Similarly, there should be an appropriate error message on a nonexistent environment variable.)

[barnabasdomozi]

@Discookie

Not sure if loading arbitrary data is intentional, but it would be useful to offload eg. the entire dictionary-based auth dictionary into the secrets, instead of each individual password.

It was initially designed for strings, but as you mentioned, it can be a useful feature so I think we can keep this as is.

(Similarly, there should be an appropriate error message on a nonexistent environment variable.)

Currently, the server prints the thrown exception message:

Variable '$ENV:FOO$' could not be resolved in server configuration file.

I think this explains that env variable FOO doesn't exist.

[Discookie]

It'd be nice to have an example about secrets.json storing JSON structures in the docs, but otherwise LGTM.

[barnabasdomozi]

@Discookie @bruntib

It'd be nice to have an example about secrets.json storing JSON structures in the docs, but otherwise LGTM.

Also extended the docs with an example dictionary secret.