Fix a too long request in metadata Admin interface (dumping all acts)

arkindex/documents/admin.py

@@ -42,7 +42,8 @@ class DateInline(admin.TabularInline):
 
 class MetaDataAdmin(admin.ModelAdmin):
     list_display = ('id', 'type', 'revision')
-    readonly_fields = ('id', 'revision', )
+    readonly_fields = ('id', 'revision')
+    raw_id_fields = ('element', )
     inlines = (DateInline, )