Allow ~user in universal ID if it's the current user

[MetRonnie]

This is a small change with no associated Issue.

Recently I encountered this

$ cylc stop '~rdutta/myworkflow'
UserInputError: Operating on others users' workflows is not supported

This PR makes it so you can pass in a full ID if the username is yours.

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).
• Applied any dependency changes to both setup.cfg and conda-environment.yml.
• Appropriate tests are included (unit)
• No change log entry needed.
• No documentation update required.

[oliver-sanders]

(no changelog needed)