Allow users/merchants to enter a sandbox API token for testing

[Noah-Silvera]

Currently the Taxjar api token is passed through an environment variable. We also need to allow consumers of this extension to enter their Taxjar sandbox API token in an easy way.

This could be accomplished through an UI on the solidus admin. Currently, no UI exists for the extension on the solidus admin side, so this ticket is a good candidate for also creating that basic admin interface.

[jarednorman]

#32 begins adding some UI.

[jarednorman]

Solidus provides a preferences system that might be of help here. Typically we prefer to statically define things, but for situations like this it can be convenient to give users the ability store these things in the UI. Would probably want to rely on encrypted preferences if available.

[Noah-Silvera]

Pulled this out of the certification milestone as it is an optional requirement. It's also possible to set via an environment variable at the moment, so we may actually meet the requirement regardless. We shall see when we enter the first round of certification and potentially revisit it down the line.