Style file upload fields

Problem/Motivation

Follow-up issue split off from 🐛 Style long text fields and text format dropdowns Fixed

Drupal's file upload element doesn't match Bootstrap 4 button styles when using a radix theme or subtheme.

Proposed resolution

Standard web browser file upload buttons are actually impossible to theme. Even default bootstrap 4 doesn't follow its own design patterns). Therefore, there is an opportunity to provide a javascript-driven Bootstrap-4 compatible solution like one of these:

1. A single button solution

   <label class="file-upload btn btn-primary">
     Choose file ... <input type="file" />
   </label>
   <script>
       $(document).ready(function() {
           $('.file-upload').file_upload();
       });
   </script>
   

2. Or an inline input group solution

   <div class="input-group">
     <div class="input-group-prepend">
       <span class="input-group-text" id="edit-profile-image-label">Upload</span>
     </div>
     <div class="custom-file">
       <input type="file" class="custom-file-input" id="edit-profile-image"
         aria-describedby="edit-profile-image-label">
       <label class="custom-file-label" for="inputGroupFile01">Choose file</label>
     </div>
   </div>