Move LemmyContext into lemmy_utils crate

[Nutomic]

Requirements

• Is this a bug report? For questions or discussions use https://lemmy.ml/c/lemmy_support
• Did you check to see if this issue already exists?
• Is this only a single bug? Do not put multiple bugs in one issue.
• Do you agree to follow the rules in our Code of Conduct?
• Is this a backend issue? Use the lemmy-ui repo for UI / frontend issues.

Summary

This way context can be used in the entire codebase, at the moment it is only available in lemmy_api_common and crates that depend on it. Then we can make Settings private, and only expose it through context instead of a global constant.