Add extraction of course_id from a block-based URL.

[doctoryes]

https://openedx.atlassian.net/browse/DENG-522

At some point in August, the format of most course-based URLs changed from:

https://courses.edx.org/courses/course-v1:org+course+run/courseware/unit1/der_3-sequential/?activate_block_id=block-v1%3Aorg%2Bcourse%2Brun%2Btype%40sequential%2Bblock%40der_3-sequential

to:

https://courses.edx.org/xblock/block-v1:org+course+run+type@vertical+block@3848270e75f34e409eaad53a2a7f1da5?show_title=0&show_bookmark_button=0

This PR adds the parsing of course_ids from the "xblock"-based course URLs which are in events. These course_ids are currently ending up as "" (empty string) and are being ignored by the pipeline code.

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Make sure that the following steps are done before merging:

• [ ] If you have a migration please contact data engineering team before merging.
• [ ] Before merging run full acceptance tests suite and provide URL for the acceptance tests run.
• [ ] A member of data engineering team has approved the pull request.

[codecov[bot]]

Codecov Report

Merging #836 into master will increase coverage by 0.00%. The diff coverage is 90.47%.

@@           Coverage Diff           @@
##           master     #836   +/-   ##
=======================================
  Coverage   74.01%   74.02%           
=======================================
  Files         208      208           
  Lines       23861    23890   +29     
=======================================
+ Hits        17661    17684   +23     
- Misses       6200     6206    +6     

Impacted Files	Coverage Δ
edx/analytics/tasks/insights/user_activity.py	73.61% <20.00%> (-1.39%)	:arrow_down:
edx/analytics/tasks/util/opaque_key_util.py	100.00% <100.00%> (ø)
edx/analytics/tasks/util/tests/test_eventlog.py	100.00% <100.00%> (ø)
...analytics/tasks/util/tests/test_opaque_key_util.py	94.00% <100.00%> (-6.00%)	:arrow_down:

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[doctoryes]

FYI: Soon I plan to execute the plan described here:

https://openedx.atlassian.net/browse/DENG-522?focusedCommentId=498316

where I'll test this branch on actual production data, producing actual production results.

The day I've chosen to re-run is 2020-08-24, which was right before the MFE URL began to be phased in. Depending on the results of that run, I'll make further changes -or- merge and run on more days.

[doctoryes]

@pwnage101 @brianhw I've removed my test code from this branch and I'm ready for a re-review.