Cylc review fixes.skip links to source

[wxtim]

(dependent on #4233 )
These changes close #4170 and a minor issue without a ticket.

• review: hide _cylc-install directory #4170 Prevent Cylc Review from following links back to source directory for workflows installed using Cylc Install.
• Prevent Cylc Review from Displaying the runN symlink to the latest run folder.

[marked against a Cylc 8 milestone because this change only adds support for Cylc 8 workflows, and should not change an aspect of Cylc 7 functioning]

• 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).
• Does not need tests: legacy product, manual testing only.
• No change log entry included & No documentation update required: Removing things that we don't want users to see before Gamma release.

To test

Install a workflow with Cylc 8.
Switch to Cylc 7 and start Cylc Review (cylc review start).
Ensure that you cannot see links to ~/cylc-run/<workflow>/_cylc-install or _cylc_install.
Ensure that you cannot see links to ~/cylc-run/<workflow>/runN

[oliver-sanders]

Bumped to 7.8.x (otherwise it will show up as closed against the wrong codebase).

[oliver-sanders]

Travis tests still working 👍.

[oliver-sanders]

Tested as working.

In the process I noticed that a running Cylc 8 workflow was marked at "stopped", not sure if that's been spotted.

[datamel]

I have checked out this branch and read the code. Tested as working. One minor typo spotted that pre-dates this pr. Not important enough to withhold merge though.