Add +/- buttons in scale deployment popup screen (#1191)

* 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>

__mocks__/@linguiMacro.ts

@@ -0,0 +1,3 @@
+module.exports = {
+  Trans: ({ children }: { children: React.ReactNode }) => children,
+};

__mocks__/electron.ts

@@ -13,5 +13,8 @@ module.exports = {
       getPath: jest.fn()
     }
   },
-  dialog: jest.fn()
+  dialog: jest.fn(),
+  ipcRenderer: {
+    on: jest.fn()
+  }
 };