[MM-22810] Update loading screen with new design & animation (#1409)

* Update loading screen with new design & animation

* add prop back in

* adjust z-index for tests

* tweaks to pass tests

* address offline feedback

- shrink initial logo size
- introduce a slight delay before fading loading spinner out
- fix horizontal scrollbar showing on load screen

* add missing css variable

* no need to remove loading icon

* Apply suggestions from code review

Co-authored-by: Guillermo Vayá <guillermo.vaya@mattermost.com>

* Move LoadingScreen.jsx to file-only component

* Rename prop for better clarity

* Default prop to none and check when needed

* Update import paths

* Add ESDocs and remove unecessary conditional

* Forgot to remove the eslint override

Co-authored-by: Guillermo Vayá <guillermo.vaya@mattermost.com>

src/package.json

@@ -12,6 +12,7 @@
     "@hapi/joi": "^16.1.8",
     "auto-launch": "^5.0.5",
     "bootstrap": "^3.3.7",
+    "classnames": "^2.2.6",
     "electron-context-menu": "^0.16.0",
     "electron-devtools-installer": "^2.2.4",
     "electron-is-dev": "^1.0.1",