add support to simply load xblock using the same host + sending extra data to student_view

This PR is to:

* Use jquery.xblock in the LMS. So, not having to specify the sessionsId and domain make it easier. We just need to specify {useCurrentHost: true}
* We can also add a data attribute in the options. Those will simply be send as get parameter to the student_view.

Yep - merging without it since we said we would keep tests for the end, but for side PRs like this it would be good to do the tests at the same time as the code.

For now, can you add this to the list of things to improve next week when you create the card?