Avoid requiring `jupyter_config.py` in to be in `uiserver` subdirectory

[wxtim]

Closes #565

Previously it was assumed to be in CYLC_SITE_CONF_PATH/uiserver.

Ensure that user is warned if file at CYLC_SITE_CONF_PATH is

• Unreadable
• Non-existent

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] CHANGES.md entry included if this is a change that can affect users
• [x] Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• [x] If this is a bug fix, PR should be raised against the relevant ?.?.x branch.