block outline at select label

Looks like material-ui's `<Select variant="outlined">` needs a redundant
label property to make the layout correct.

https://next.material-ui.com/api/outlined-input/#main-content

"The label of the input. It is only used for layout. The actual
labelling is handled by InputLabel. If specified labelWidth is ignored."

ui/src/List/DisplaySelector.tsx

@@ -49,6 +49,7 @@ const DisplaySelector = (props: Props) => {
         </InputLabel>
         <Select
           labelId="split90k-label"
+          label="Max video duration"
           id="split90k"
           value={props.split90k}
           onChange={(e) => props.setSplit90k(e.target.value)}