Refactor GraphQL

[MetRonnie]

These changes partially address #271

Sibling: cylc/cylc-flow#4529

Ensure consistency between Resolvers.mutator() in cylc.flow.network.resolvers and cylc.uiserver.resolvers to avoid traceback issues like in #271

Requirements check-list

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Appropriate tests are included (unit and/or functional).
• Appropriate change log entry included.
• No documentation update required
• No dependency changes.