Fix dashboard report image download (edx-release hotfix)

[omarkhan]

The dashboard report image download can currently fail in 2 different ways:

• If the image is from a different domain and it has been cached by the browser, it won't be converted into a data URL due to cross-domain security issues. A javascript SecurityError is thrown, and nothing happens.
• If the image url starts with // (no explicit protocol) or the url is relative (e.g. /static/...), and it hasn't yet been cached by the browser, it will fail to show up in the downloaded report.

This pull request fixes both issues, by:

• Not trying to create a data URL for cross-domain requests
• Expanding relative urls and urls without an explicit protocol

[omarkhan]

The tests are failing because the version of xblock-sdk this is using does not include the version of bok choy that allows us to test screenshots. @bradenmacdonald shall I just remove the tests from this hotfix PR and put them in a PR against master?

[bradenmacdonald]

If your test relies on an an external image, it should be hosted on GitHub.

[bradenmacdonald]

@omarkhan Sure, if you want to use screenshot tests, you can include them only in the master version.

For the screenshots, is it possible to include some colorization in one of the segments of the image? That would be a more comprehensive test. (See screenshot I attached to the OC ticket).

[omarkhan]

@bradenmacdonald I have removed the tests from this PR, but the existing tests are failing on circleci. I did some digging and it looks like none of the test setup (installing test requirements, using the run_tests.py wrapper script) is happening on circleci. Is there a reason we are using circleci as well as travis? Should I fix the circleci tests or just drop them?

[omarkhan]

@bradenmacdonald I have opened another PR for master that includes tests, including a test for the colorized overlays: #107

[bradenmacdonald]

@omarkhan We use CircleCI on master and Travis on edx-release. Eventually we will move both to CircleCI.

[omarkhan]

@bradenmacdonald then I guess this is ready to go, once I get your +1

[bradenmacdonald]

@omarkhan Nice fix. Code LGTM. Haven't tested it. @Kelketek is your reviewer and will give you +1.

[Kelketek]

👍

[omarkhan]

@tobz this is ready for upstream review, can you take a look? Thanks!

[tobz]

LGTM. 👍