Automatically fill description when importing YouTube video.

[Leftium]

• The description will usually be based on the description from the YouTube video.
• It is easier to delete text then copy paste text.
• For example, take https://youtu.be/_867JAoBh7s:

We're putting the band back together. Svelte Radio is officially back. This first episode is a bit of catch-up. Not a ton of Svelte chatter. Look forward to many fun and interesting episodes.

To reproduce

1. Import a YouTube video using https://prod.sveltesociety.dev/admin/external-content
2. Edit the freshly imported content.

Expected

• Description field is automatically filled based on YouTube video.

Actual

• Description field is empty.