feat: use custom intro/outro playlists per show (#2941)

### Description

Having a global intro and outro playlist in settings is not very
flexible for special programming. This adds an override intro/outro
playlist per show. If it is not set, the global one is used. also it's
ignored if there's no autloading at all.

**I have updated the documentation to reflect these changes**:

Yes

### Testing Notes

**What I did:**
Schedule 2 shows, one without defining custom lists, one with defining
custom lists. one hour before the show starts it should be populated
correctly. If you define a global list it shojuld be replaced with the
per-show list.

---------

Co-authored-by: Thomas Göttgens <[email protected]>
libretime/libretime@299be3c

schema.yml

@@ -6409,6 +6409,12 @@ components:
           type: boolean
         auto_playlist_repeat:
           type: boolean
+        intro_playlist:
+          type: integer
+          nullable: true
+        outro_playlist:
+          type: integer
+          nullable: true
     PatchedShowDays:
       type: object
       properties:
@@ -7241,6 +7247,12 @@ components:
           type: boolean
         auto_playlist_repeat:
           type: boolean
+        intro_playlist:
+          type: integer
+          nullable: true
+        outro_playlist:
+          type: integer
+          nullable: true
       required:
         - auto_playlist_enabled
         - auto_playlist_repeat