Show DiscSubtitle even if the album has only one disc.

Closes #947

ui/src/common/SongDatagrid.js

@@ -52,6 +52,15 @@ const DiscSubtitleRow = ({
   const handlePlayDisc = (discNumber) => () => {
     onClick(discNumber)
   }
+
+  let subtitle = []
+  if (record.discNumber > 0) {
+    subtitle.push(record.discNumber)
+  }
+  if (record.discSubtitle) {
+    subtitle.push(record.discSubtitle)
+  }
+
   return (
     <TableRow
       hover
@@ -61,8 +70,7 @@ const DiscSubtitleRow = ({
       <TableCell colSpan={colSpan}>
         <Typography variant="h6" className={classes.subtitle}>
           <AlbumIcon className={classes.discIcon} fontSize={'small'} />
-          {record.discNumber}
-          {record.discSubtitle && `: ${record.discSubtitle}`}
+          {subtitle.join(': ')}
         </Typography>
       </TableCell>
       <TableCell>
@@ -148,11 +156,13 @@ const SongDatagridBody = ({
     if (!ids) {
       return new Set()
     }
+    let foundSubtitle = false
     const set = new Set(
       ids
         .filter((i) => data[i])
         .reduce((acc, id) => {
           const last = acc && acc[acc.length - 1]
+          foundSubtitle = foundSubtitle || data[id].discSubtitle
           if (
             acc.length === 0 ||
             (last && data[id].discNumber !== data[last].discNumber)
@@ -162,7 +172,7 @@ const SongDatagridBody = ({
           return acc
         }, [])
     )
-    if (!showDiscSubtitles || set.size < 2) {
+    if (!showDiscSubtitles || (set.size < 2 && !foundSubtitle)) {
       set.clear()
     }
     return set