validate thumbnail is URL

client/homebrew/editor/metadataEditor/metadataEditor.jsx

@@ -275,7 +275,7 @@ const MetadataEditor = createClass({
 						<label>thumbnail</label>
 						<input type='text'
 							defaultValue={this.props.metadata.thumbnail}
-							placeholder='my.thumbnail.url'
+							placeholder='https://my.thumbnail.url'
 							className='value'
 							onChange={(e)=>this.handleFieldChange('thumbnail', e)} />
 						<button className='display' onClick={this.toggleThumbnailDisplay}>

client/homebrew/editor/metadataEditor/validations.js

@@ -12,6 +12,14 @@ module.exports = {
 	thumbnail : [
 		(value)=>{
 			return value?.length > 256 ? 'Max URL length of 256 characters.' : null;
+		},
+		(value)=>{
+			try {
+				Boolean(new URL(value));
+				return null;
+			} catch (e) {
+				return 'Must be a valid URL';
+			}
 		}
 	],
 	language : [