more improvements, fix url, do the same for cover
remember url, error if invalid
unneeded addition
Fix delayed message
Lint
Allow empty values (placeholder and Gerbil)
Fix filepath crash
Fix button
These only work in the `selectThumbnail` component. If it's small enough, it'll get past the transaction limit-check, but ultimately it doesn't appear correctly in the claim list.
It'll probably appear correctly if we tweak the ClaimPreview code, but since this also fails the Google Video metadata test, I think we should disallow this.
## Issue
5923
Thumbnail errors were just being used in `selectThumbnail`. The form doesn't know about it and was using allowing invalid thumbnails like 'helloworld' to pass through.
