Fix visual regressions around widget permissions (#10954)

* Add a Jest snapshot of AppPermission

* Move the test inside 'for a pinned widget' category

* Make only spinner message bold

* Set font size specified with "mx_AppPermission_smallText" by default

- Add "mx_AppPermission_largeText" for elements whose size has not been specified with mx_AppPermission_smallText
- Create _AppWarning.pcss for AppWarning

* Make AppPermission panel scrollable, keeping the content at the center

* Run prettier

* Use Heading component

* Use Icon component

* Fix the test

res/img/feather-customised/help-circle.svg

@@ -1,5 +1,5 @@
 <svg xmlns="http://www.w3.org/2000/svg" width="14" height="14" viewBox="0 0 14 14">
-    <g fill="none" fill-rule="evenodd" stroke="#454545" stroke-linecap="round" stroke-linejoin="round" transform="translate(1 1)">
+    <g fill="none" fill-rule="evenodd" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" transform="translate(1 1)">
         <circle cx="6" cy="6" r="6"/>
         <path d="M4.254 4.2a1.8 1.8 0 0 1 3.498.6c0 1.2-1.8 1.8-1.8 1.8M6 8.991"/>
     </g>