Fixes #12791 - Use a selector to only select the correct tax rate sel…

[hostep]

…ect element on the tax rule edit page, instead of all three selects on that page.

Description

See description on #12791 for more info.

Short summary: on the tax rule edit page in the backend, there are 3 elements which are selected with the default selector section.mselect-list:

• Tax Rate multiselect
• Customer Tax Class multiselect
• Product Tax Class multiselect

Only the first one should be selected, otherwise really weird things happen on that page where the other 2 elements somehow are getting involved with actions you perform on the first multiselect.
You can only reproduce this behavior when certain javascript code is being delayed a bit while executing, like sometimes happens on a production server where not all assets are cached yet in your browser.

The proposed fix is to use a more specific selector: #tax_rate + section.mselect-list which only selects the correct container following the Tax Rate multiselect.

Would be nice if someone could do some rigorous testing, because I had very little time yet to test this fix properly.

Fixed Issues (if relevant)

1. Customer & Product Tax class wrongly styled #12791: Customer & Product Tax class wrongly styled

Manual testing scenarios

1. See Customer & Product Tax class wrongly styled #12791 (comment)

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-engcom-team]

Hi @okorshenko, thank you for the review.
MAGETWO-87941 has been created to process this Pull Request