fix: video thumbnail availability (#421)

src/editors/containers/VideoEditor/components/VideoSettingsModal/components/ThumbnailWidget/__snapshots__/index.test.jsx.snap

@@ -22,7 +22,7 @@ exports[`ThumbnailWidget snapshots snapshots: renders as expected where thumbnai
     variant="light"
   >
     <FormattedMessage
-      defaultMessage="Select a video from your library to enable this feature"
+      defaultMessage="Select a video from your library to enable this feature (applies only to courses that run on the edx.org site)."
       description="Message for unavailable thumbnail widget"
       id="authoring.videoeditor.thumbnail.unavailable.message"
     />
@@ -167,6 +167,15 @@ exports[`ThumbnailWidget snapshots snapshots: renders as expected with a thumbna
       id="authoring.videoeditor.thumbnail.error.fileSizeError"
     />
   </ErrorAlert>
+  <Alert
+    variant="light"
+  >
+    <FormattedMessage
+      defaultMessage="Select a video from your library to enable this feature (applies only to courses that run on the edx.org site)."
+      description="Message for unavailable thumbnail widget"
+      id="authoring.videoeditor.thumbnail.unavailable.message"
+    />
+  </Alert>
   <Stack
     direction="horizontal"
     gap={3}
@@ -204,7 +213,7 @@ exports[`ThumbnailWidget snapshots snapshots: renders as expected with default p
     variant="light"
   >
     <FormattedMessage
-      defaultMessage="Select a video from your library to enable this feature"
+      defaultMessage="Select a video from your library to enable this feature (applies only to courses that run on the edx.org site)."
       description="Message for unavailable thumbnail widget"
       id="authoring.videoeditor.thumbnail.unavailable.message"
     />

src/editors/containers/VideoEditor/components/VideoSettingsModal/components/ThumbnailWidget/index.jsx

@@ -75,7 +75,7 @@ export const ThumbnailWidget = ({
       >
         <FormattedMessage {...messages.fileSizeError} />
       </ErrorAlert>
-      {edxVideo ? null : (
+      {(allowThumbnailUpload && edxVideo) ? null : (
         <Alert variant="light">
           <FormattedMessage {...messages.unavailableMessage} />
         </Alert>
@@ -90,7 +90,7 @@ export const ThumbnailWidget = ({
             src={thumbnailSrc || thumbnail}
             alt={intl.formatMessage(messages.thumbnailAltText)}
           />
-          { (allowThumbnailUpload && edxVideo) ? (
+          {(allowThumbnailUpload && edxVideo) ? (
             <IconButtonWithTooltip
               tooltipPlacement="top"
               tooltipContent={intl.formatMessage(messages.deleteThumbnail)}
@@ -115,7 +115,7 @@ export const ThumbnailWidget = ({
             iconBefore={FileUpload}
             onClick={fileInput.click}
             variant="link"
-            disabled={!edxVideo}
+            disabled={!(allowThumbnailUpload && edxVideo)}
           >
             <FormattedMessage {...messages.uploadButtonLabel} />
           </Button>

src/editors/containers/VideoEditor/components/VideoSettingsModal/components/ThumbnailWidget/messages.js

@@ -24,7 +24,8 @@ const messages = defineMessages({
   },
   unavailableMessage: {
     id: 'authoring.videoeditor.thumbnail.unavailable.message',
-    defaultMessage: 'Select a video from your library to enable this feature',
+    defaultMessage:
+      'Select a video from your library to enable this feature (applies only to courses that run on the edx.org site).',
     description: 'Message for unavailable thumbnail widget',
   },
   uploadButtonLabel: {