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/css/views/rooms/_AppsDrawer.pcss

@@ -311,14 +311,7 @@ limitations under the License.
     display: flex;
     height: 100%;
     flex-direction: column;
-    justify-content: center;
     align-items: center;
-    font-size: $font-16px;
-
-    h4 {
-        margin: 0;
-        padding: 0;
-    }
 }
 
 .mx_AppTile_loading {
@@ -326,7 +319,6 @@ limitations under the License.
     flex-direction: column;
     justify-content: center;
     align-items: center;
-    font-weight: bold;
     position: relative;
     height: 100%;