Seed tax rate in dummy app and sandbox

[Noah-Silvera]

Currently, the solidus app created does not have a tax rate configured in the sandbox or dummy app.

This is blocking some tests in #139, and also makes the sandbox environment not representative of a solidus store that this extension would be run in.

We should follow the steps in the readme when generating the dummy app and sandbox app

Finally, make sure that the TAXJAR_API_KEY environment variable is set to your TaxJar API key and make sure that you have a Spree::TaxRate with the name "Sales Tax". This will be used as the source for the tax adjustments that Solidus creates.

[Noah-Silvera]

cc: @forkata @benjaminwil