[21.01] Shared visualization display fix

[dannon]

What did you do?

• Use full page context (not just the short embed chunk) when injecting a viz in an iframe

Why did you make this change?

Fixes #11888

How to test the changes?

• Instructions for manual testing are as follows:
  1. See Shared visualisation displays only a small section #11888, create and view a visualization (basic chart like nvd3 will work fine) like that.

License

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[simonbray]

Thanks for taking a look @dannon!

[dannon]

@simonbray Thanks for the report! And thanks for the merge @martenson.