Improve file upload experience in content editor

...javascripts/batch_comments/components/submit_dropdown.vue

...ntent_editor/components/bubble_menus/link_bubble_menu.vue

...tent_editor/components/bubble_menus/media_bubble_menu.vue

.../javascripts/content_editor/components/content_editor.vue

...cripts/content_editor/components/content_editor_alert.vue

...s/content_editor/components/toolbar_attachment_button.vue

...ssets/javascripts/content_editor/extensions/attachment.js

app/assets/javascripts/content_editor/extensions/link.js

app/assets/javascripts/content_editor/extensions/loading.js

app/assets/javascripts/content_editor/extensions/playable.js

...ascripts/content_editor/services/create_content_editor.js

...avascripts/content_editor/services/markdown_serializer.js

...ascripts/content_editor/services/serialization_helpers.js

...ets/javascripts/content_editor/services/upload_helpers.js

app/assets/javascripts/content_editor/services/utils.js

app/assets/javascripts/lib/utils/tappable_promise.js

...ets/javascripts/notes/components/comment_field_layout.vue

...sets/javascripts/vue_shared/components/markdown/field.vue

...cripts/vue_shared/components/markdown/markdown_editor.vue

app/assets/stylesheets/components/content_editor.scss