* Add @testing-library/jest-dom and @testing-library/react for writing test cases

* Make eslint check __mocks__/*.ts files

* Mock electron.ipcRenderer.on to prevent errors in React compoment test cases

* Mock <Trans /> from @lingui/macro as a component simplely returns children components

* Use optional chaining operator (?.) to avoid TypeError: Cannot read property 'replicas' of undefined

* Add jest-fetch-mock for mocking global.fetch, activate for all test cases in jest.setup.ts

* Add basic tests for <DeploymentScaleDialog /> to ensure it initialises/renders without errors

* Add +/- button for changing desired replicas, and its test cases

* Adjust the styles of +/- buttons container to make sure it aligns with the slider

* Refactor after rebase from master

* Use var defined in vars.scss

* Add flex align-center to slider-container and remove margin-top: -4px

* Use <Icon /> to replace <i/>

* Add flex gaps to plus-minus-container
Signed-off-by: Hung-Han (Henry) Chen <1474479+chenhunghan@users.noreply.github.com>