Edit Tax sourcing

[derBretti]

Resolves #257 , #259
Impact: minor
Type: feature

Issue

1. Tax sourcing was not editable.
2. Addresses were not matched after editing region or postal code in custom tax rate.

Solution

1. Add field to editor and column to table.
2. Replace emtpy strings with null in postal code and region when creating or updating a custom tax rate.

Breaking changes

none

Testing

1. Go to settings/tax-settings
2. Note additional sourcing column
3. Add or edit a custom tax rate, set value for region and postal code
4. Note dropdown for sourcing field
5. Edit a custom tax rate, remove values for region and postal code
6. Note that the edited tax rate is applied when only the country matches

[aldeed]

Might also fix reactioncommerce/reaction#6231

[loan-laux]

Just noticed this PR. I've actually just submitted api-plugin-taxes-flat-rate#3 to fix this issue. It also fixes reaction#6231.

I think @derBretti's solution of sending null values instead of empty strings right from reaction-admin is cleaner. My solution was to default to null in case an empty string was received, but sending the right value from the beginning is a lot better.

@aldeed let me know if you want me to remove this server-side value check, since @derBretti's changes will be sufficient.

However, PR alone won't fix reaction#6231, since the $or selector in the MongoDB query for the Taxes collection needed to be updated to allow for country: null too.

[loan-laux]

@willopez While you're at it, it would be great if you could also review and merge this one that's been sitting here for a while.

[willopez]

Looks good, thanks @derBretti!