Allow setting user conf path through environment variables

[ScottWales]

Teach the uiserver about two environment variables:

$CYLC_CONF_PATH (default ~/.cylc) sets a path to read the uiserver configuration from, and also allows USER_CONF_ROOT in the server configuration to be updated by users. This variable matches the behaviour of cylc-flow.

$CYLC_UISERVER_LOG_PATH (default ~/.cylc/uiserver) sets the output path for Cylc uiserver logs. These logs are created before the jupyter configuration is loaded so are unable to be configured there.

Closes #489

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
• [ ] 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.