Wait for js/base in CMS acceptance tests to ensure click handlers are installed

This should help with a rare failure seen in master:

https://jenkins.testeng.edx.org/job/edx-all-tests-auto-master/229/SHARD=3,TEST_SUITE=cms-acceptance/testReport/junit/CMS/Video%20Component%20_%20When%20start%20end%20end%20times%20are%20specified,%20a%20range%20on%20slider%20is%20shown/Given_I_have_created_a_Video_component/

From what I can tell, we weren't waiting for `base.js` to load, so click handlers weren't getting installed.  This update adds an explicit wait for `base.js` to all Studio pages; it also waits for JavaScript to load before clicking the section expand toggle in `create_course_with_unit()`

@jzoldak