fix(background): when list background is removed, delete file from file system and DB

Currently, if a list background is removed, the background file stays in disk and in the DB forever.

Fix this by deleting the file during background removal, similar to how it's done when a new background is uploaded: [source](726a517bec/pkg/modules/background/upload/upload.go (L56-L62))

Test Plan:
A) Before these changes
A.1) Create a new list
A.2) Set a new background to the list
A.3) Check file is in disk and DB:
```
$ ls files/
1

$ sqlite3 vikunja.db 
SQLite version 3.40.1 2022-12-28 14:03:47
Enter ".help" for usage hints.
sqlite> select * from files;
1|20220410_180833.jpg||2822966|2023-01-25 21:25:28|1
sqlite> select * from lists;
1|Teste||||1|1|0|1|LeGlCmIqR*of_4R+WCoe4;s.oLjZ|65536.0|2023-01-25 21:25:21|2023-01-25 21:25:28
```

A.4) Remove background from list
A.5) Check file is still in disk and DB:
```
$ ls files/
1

$ sqlite3 vikunja.db 
SQLite version 3.40.1 2022-12-28 14:03:47
Enter ".help" for usage hints.
sqlite> select * from files;
1|20220410_180833.jpg||2822966|2023-01-25 21:25:28|1
sqlite> select * from lists;
1|Teste||||1|1|0|0||65536.0|2023-01-25 21:25:21|2023-01-25 21:32:39
```

B) After these changes
B.1) Set background again
B.2) Check files in disk and DB:
```
$ ls files/
1 2

$ sqlite3 vikunja.db 
SQLite version 3.40.1 2022-12-28 14:03:47
Enter ".help" for usage hints.
sqlite> select * from files;
1|20220410_180833.jpg||2822966|2023-01-25 21:25:28|1
2|20220410_180833.jpg||2822966|2023-01-25 21:34:33|1
sqlite> select * from lists;
1|Teste||||1|1|0|2|LeGlCmIqR*of_4R+WCoe4;s.oLjZ|65536.0|2023-01-25 21:25:21|2023-01-25 21:34:33
```

B.3) Remove background image
B.4) Check files and DB again, verify image 2 has been deleted:
```
$ ls files/
1

$ sqlite3 vikunja.db 
SQLite version 3.40.1 2022-12-28 14:03:47
Enter ".help" for usage hints.
sqlite> select * from files;
1|20220410_180833.jpg||2822966|2023-01-25 21:25:28|1
sqlite> select * from lists;
1|Teste||||1|1|0|0||65536.0|2023-01-25 21:25:21|2023-01-25 21:36:13
```

@@ -332,1 +332,4 @@
 	}
+	
+	// Remove the old background if one exists
+	if list.BackgroundFileID != 0 {

@@ -260,6 +261,40 @@ func TestList_Delete(t *testing.T) {
 	})
 }
 
+func TestList_DeleteBackgroundFileIfExists(t *testing.T) {

@@ -815,12 +815,28 @@ func (l *List) Delete(s *xorm.Session, a web.Auth) (err error) {
 		}
 	}
 
+	err = l.DeleteBackgroundFileIfExists()

@@ -795,6 +795,11 @@ func (l *List) Create(s *xorm.Session, a web.Auth) (err error) {
 // @Router /lists/{id} [delete]
 func (l *List) Delete(s *xorm.Session, a web.Auth) (err error) {
 
+	err = l.DeleteBackgroundFileIfExists(s)

@@ -260,0 +265,4 @@
+		db.LoadAndAssertFixtures(t)
+		files.InitTestFileFixtures(t)
+		s := db.NewSession()
+		list, err := GetListSimpleByID(s, 25)