report-note: admin: Add a link to relevant song

Previously, the admin could not directly move from a report note to
its corresponding song. This commit adds a link that will go
directly to the "change" page for the corresponding song.

marietje/songs/admin.py

@@ -1,4 +1,6 @@
 from django.contrib import admin
+from django.urls import reverse
+from django.utils.html import format_html
 
 from .models import ReportNote, Song
 
@@ -32,4 +34,11 @@ class SongAdmin(admin.ModelAdmin):
 class ReportNoteAdmin(admin.ModelAdmin):
     list_display = ('song', 'note', 'user')
     search_fields = ('song__artist', 'song__title', 'user__name')
-    readonly_fields = ('song',)
+    readonly_fields = ('song_link',)
+
+    @staticmethod
+    def song_link(note):
+        url = reverse("admin:songs_song_change", args=(note.song.id,))
+        return format_html("<a href='{url}'>{song}</a>", url=url, song=note.song)
+
+    song_link.short_description = "Song link"