Add support for self hosted Tableau

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Review/Testing instructions for the merge request:

1. Configure allowed hosts at /admin/config/media/tableau.
2. Create a Tableau media item with the embed hosted at one of the allowed hosts entered above.
3. Test with both self and publicly (https://public.tableau.com) hosted domains.

I left two small comments in the PR, but I tested it locally and it's working great! existing visualizations pointing to the public domain keeps working, and if I add another domain, it works well too.

I tested this MR in my local and it's working as expected, thanks!!

Automatically closed - issue fixed for 2 weeks with no activity.