[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>

.eslintrc.json

@@ -24,7 +24,9 @@
     "react/jsx-indent-props": [2, 2],
     "react/no-find-dom-node": 2,
     "react/no-set-state": 1,
-    "react/require-optimization": 0
+    "react/require-optimization": 0,
+    "multiline-ternary": ["warn", "always-multiline"],
+    "consistent-return": "off"
   },
   "overrides": [
     {