metadata view: introduce a minimal task metadata view

[oliver-sanders]

Addresses #582

• Bare bones task metadata view.
• Display task metadata, prerequisites and outputs.
• Display / functionality improvements and support for other information to follow in future PRs.

Check List

• [x] I have read CONTRIBUTING.md and added my name as a Code Contributor.
• [x] Contains logically grouped changes (else tidy your branch by rebase).
• [x] Does not contain off-topic changes (use other PRs for other changes).
• [x] Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• [x] Tests are included (or explain why tests are not needed).
• [x] Changelog entry included if this is a change that can affect users
• [ ] Cylc-Doc - bumped changes entry to a PR (view is likely to change further before release)
• [x] If this is a bug fix, PR should be raised against the relevant ?.?.x branch.