[Forwardport] local themes should be added to git repo

[torhoehn]

Original Pull Request

#18960

Description (*)

Since local-themes.js is always created manually by copying themes.js it should be added to git. So themes.js could also be a .js.dist-file which has to be copied and would be added to git, too.

Fixed Issues (if relevant)

1. dev/tools/grunt/configs/themes.js gets replaced after update magento #18949: dev/tools/grunt/configs/themes.js gets replaced after update magento

Manual testing scenarios (*)

1. Copy dev/tools/grunt/configs/themes.js to dev/tools/grunt/configs/local-themes.js
2. local-themes.js should be found by git now

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 @torhoehn. Thank you for your contribution
Here is some useful tips how you can test your changes using Magento test environment.
Add the comment under your pull request to deploy test or vanilla Magento instance:

• @magento-engcom-team give me test instance - deploy test instance based on PR changes
• @magento-engcom-team give me 2.3-develop instance - deploy vanilla Magento instance

For more details, please, review the Magento Contributor Assistant documentation

[magento-engcom-team]

Hi @sivaschenko, thank you for the review.
ENGCOM-3538 has been created to process this Pull Request

[magento-engcom-team]

Hi @torhoehn. Thank you for your contribution.
We will aim to release these changes as part of 2.3.1.
Please check the release notes for final confirmation.