[Improvement] Implement customer group form on ui component

[simpleadm]

Description

Several versions ago backend form widgets (\Magento\Backend\Block\Widget\Form) were marked as
deprecated in favour of UI component implementation. This PR refactors customer group form to use UI components. Also was added "Save and Continue Edit" button.
At least it will save time of third party developers who needs to extend customer groups form :)

Customer group grid was implemented in scope of #13501

Manual testing scenarios

Before changes:

After changes:

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)

[okorshenko]

Hi @simpleadm thank you for this improvement. We will start review process now

[simpleadm]

Please pay attention about backward compatibility in save controller and let me know if it should be fixed.

And what about integration tests for it?

[okorshenko]

Hi @simpleadm
We are ok with renaming request parameter for the controller in release 2.3.0. This might affect some tests, but we will try to fix them as well. New code must be covered with tests (unit or integration).
Please stabilize your current implementation and tests. Once branch is stable, we will proceed with testing and acceptance

[okorshenko]

Please, fix broken tests

[sidolov]

Hi @simpleadm , any updates?

[simpleadm]

Hi @sidolov ,
I will be able continue this task next week

[sidolov]

Hi @simpleadm , thanks for the information!

[simpleadm]

Sorry, hope to start this task on current week :(

[VladimirZaets]

Hi @simpleadm , do you have some update?

[sidolov]

@simpleadm , I am closing this PR now due to inactivity.
Please reopen and update if you wish to continue.
Thank you for collaboration!