UX: Add image uploader widget for uploading badge images (#12377)

Currently the process of adding a custom image to badge is quite clunky; you have to upload your image to a topic, and then copy the image URL and pasting it in a text field. Besides being clucky, if the topic or post that contains the image is deleted, the image will be garbage-collected in a few days and the badge will lose the image because the application is not that the image is referenced by a badge.

This commit improves that by adding a proper image uploader widget for badge images.

spec/requests/admin/badges_controller_spec.rb

@@ -138,6 +138,7 @@ describe Admin::BadgesController do
       it 'updates the badge' do
         SiteSetting.enable_badge_sql = true
         sql = "select id user_id, created_at granted_at from users"
+        image = Fabricate(:upload)
 
         put "/admin/badges/#{badge.id}.json", params: {
           name: "123456",
@@ -145,13 +146,17 @@ describe Admin::BadgesController do
           badge_type_id: badge.badge_type_id,
           allow_title: false,
           multiple_grant: false,
-          enabled: true
+          enabled: true,
+          image_upload_id: image.id,
+          icon: "fa-rocket",
         }
 
         expect(response.status).to eq(200)
         badge.reload
         expect(badge.name).to eq('123456')
         expect(badge.query).to eq(sql)
+        expect(badge.image_upload.id).to eq(image.id)
+        expect(badge.icon).to eq("fa-rocket")
       end
 
       context 'when there is a user with a title granted using the badge' do