Refactor use of common strings to enum

[sgaist]

Proposed change

There are several strings (user, group, server, service, etc) that are used throughout the code base as keys, for loop items and the likes. These could be considered constants and be moved to one or more enum.

Despite making the code more verbose, it would help avoiding pitfalls usually associated with typos just as wrong keys and would trigger explicit errors.

As an example, the content of the scopes.py where there's a lot of checks and data extraction using such strings.

Alternative options

Don't change anything

Who would use this feature?

Developers of JupyterHub as well as people reading through code for better understanding.

(Optional): Suggest a solution

• Analyse the strings used in the code base
• Establish one or more Enum to replace them (for example Resources for user, group, server, service.
• Update strings use with Enum.

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[manics]

I think this is worth trying on one of the proposed enum types first, so we can evaluate tradeoffs in how they're implemented. E.g.
__str__ can be overridden to return Enum.item.value instead of the default representation making it easier to use in strings, at the expense of not behaving how Enums normally behave.