Remove tabindex of -1 on upload image input in the write a post editor (#10543)

* Remove tabindex of -1 on upload image input in the write a post editor

* Remove negative tabindex on image uploader's outer button

* Give tabindex of -1 to outer button on image uploader

* Update buttons.scss

app/assets/stylesheets/components/buttons.scss

@@ -68,7 +68,8 @@
   &[href]:focus,
   &:hover:enabled,
   &:active:enabled,
-  &:focus:enabled {
+  &:focus:enabled,
+  &:focus-within {
     background-color: var(--bg-hover);
     border-color: var(--border-hover);
     box-shadow: var(--shadow-hover);

app/javascript/article-form/components/ImageUploader.jsx

@@ -146,7 +146,6 @@ export const ImageUploader = () => {
             multiple
             accept="image/*"
             data-max-file-size-mb="25"
-            tabIndex="-1"
             aria-label="Upload an image"
           />
         </Button>