[feature] Let user choose prefix of the change diff in the notification

**Version and OS**
Ubuntu 22.04

**Is your feature request related to a problem? Please describe.**
It would be useful to have to option to change the prefix of the change diff in the notification like "(added)" and "(removed)" into something more easy on the eyes like "+" and "-".

**Describe the solution you'd like**
Let user choose prefix of the change diff in the notification.

**Describe the use-case and give concrete real-world examples**
```
(added) Jan Lipold
(added) šéfkomentátor
(added) Zuzana Kubátová
(added) šéfreportérka SZ Byznys
(removed) Jindřich Šídlo
(removed) komentátor
(removed) Jiří Kubík
(removed) šéfredaktor The diff output - only changes, additions, and removals
```
->
```
+ Jan Lipold
+ šéfkomentátor
+ Zuzana Kubátová
+ šéfreportérka SZ Byznys
- Jindřich Šídlo
- komentátor
- Jiří Kubík
- šéfredaktor The diff output - only changes, additions, and removals
```

**Additional context**
Followthatpage.com has this and it's much easier to read.

Start by locating where notification change diffs are formatted and where user notification preferences are defined. Trace the existing added and removed prefixes, then determine how a selectable prefix setting would be exposed and verify that notifications use the chosen symbols consistently.

Written by the indexing model from the issue text.