Make SQL Lab explore use the default viz from the config file

[cccs-Dustin]

In Superset's config file, there is a config option called DEFAULT_VIZ_TYPE which you can use to define which viz you would like to use in the chart explorer by default. This is a very useful config option, especially when you have a custom viz you would like to use instead of the Superset Table viz. However, when you are in SQL Lab and you want to create and explore a dataset, there is currently no way to to change the default viz which will be used.

The DEFAULT_VIZ_TYPE config option should also be used by SQL Lab when you want to explore a dataset. This will allow for consistency between the chart explorer, and exploring through SQL Lab.

I have already implemented the code changes for this feature locally, and will create a PR for them. Once the PR has been made, I will link it here: #20056

[cccs-Dustin]

NOTE: This discussion can be deleted/disregarded as the PR has been merged into Master