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

docs/user-manual/scheduling-shows.md

@@ -79,6 +79,8 @@ indicator.
 | Add Autoloading Playlist?           | If checked, allows for the following options                                                                                                                                                                                                                                                                                                                                                                                               |
 | Select Playlist                     | Select the playlist the show will autofill from (shows autofill exactly one hour before air). If you wish to use a smartblock you must add it to a playlist and then select that playlist. This can be used to auto schedule new podcast episodes to air.                                                                                                                                                                                  |
 | Repeat Playlist Until Show Is Full  | If checked, the playlist will be added to the show multiple times until the slot is full. Useful for applying a one-hour music playlist made up of smartblocks to a two-hour show.                                                                                                                                                                                                                                                         |
+| Select Intro Playlist               | Select the playlist to replace the global intro playlist from settings. If you wish to use a smartblock you must add it to a playlist and then select that playlist.                                                                                                                                                                                                                                                                       |
+| Select Outro Playlist               | Select the playlist to replace the global outro playlist from settings. If you wish to use a smartblock you must add it to a playlist and then select that playlist.                                                                                                                                                                                                                                                                       |
 | _Live Stream Input_                 |                                                                                                                                                                                                                                                                                                                                                                                                                                            |
 | Use LibreTime/Custom Authentication |                                                                                                                                                                                                                                                                                                                                                                                                                                            |
 | Show Source                         |                                                                                                                                                                                                                                                                                                                                                                                                                                            |