Simplify URL construction in Selenium tests

[jooooosef]

In normal tests, we can issue requests with self.app.get(reverse(...)). However, in live server tests it looks like this:

self.selenium.get(self.live_server_url + reverse("staff:semester_view", args=[semester.pk]))

We should come up with a way to not always write self.live_server_url + .

Originally posted by @niklasmohrin in #2474 (comment)