cypress: run tests against safari

[oliver-sanders]

Run e2e tests against the webs's second most popular web browser.

Cylc UI currently works very nicely in Safari, however, must be tested manually.

Depends on Cypress implementing Safari/WebKit support - https://github.com/cypress-io/cypress/issues/6422

Pull requests welcome!

[oliver-sanders]

WebKit support has now landed in Cypress (currently an "experimental" feature):

https://docs.cypress.io/guides/guides/launching-browsers#WebKit-Experimental