Add a default company code value and improve help text

If an API request doesn't specify a companyCode, the account's default company code is used. It turns out you can also pass DEFAULT as the company code for the same effect. Our module does let you specify an alternate company code per store, but it's not very clear in the UI, and incorrectly putting in a company code causes errors.

Let's simplify things a bit:

1. Add DEFAULT as the default value for company code in the settings form.
2. Adjust the help text to, "Use DEFAULT to calculate taxes using the default company configuration in your Avalara account. Otherwise, set a specific company code to use for this site.
   If a company code is set for an order's store, that company code will be used instead to calculate the order's taxes."
3. Add help text to the company code field on the store edit form reading, "Enter a company code to use to calculate taxes for this store's orders instead of @company_code." Use the company code from the global AvaTax settings for that placeholder.