Make "Use spoilers when mentioning targets of a rule" feature optional

Nobody has claimed this yet.

Even though I understand that it makes it easier to anonymize or hide user information in screenshots, I would still argue that #559 is a hindrance for some moderators (or at least of us). If we take screenshots, even for internal use, we anonymize them ourselves. We don't need this to be a feature of the bot.

When I start my day on Matrix, the first thing I do is read what the "nightshift" did, which users were affected and why. The moderation room is part of that routine. I usually jump back and forth between rooms to get a better understanding of a situation. Or I just want to scroll back a couple of days and visually get an impression of previous events to confirm a pattern.
I basically want to read the moderation room like a log file. Wrapping mentioning targets in spoilers makes that task harder.

Another argument would be that the bot itself leaks that information under certain conditions anyway:

1. After the message which wraps the mention target in a spoiler, if sends the following message to the moderation room, which has not (yet?) been wrapped:
```plaintext
some_room:domain.tld (shortcode: my_shotcode) updated with 1 change:

ADDED user (m.ban): @some_user:domain.tld (some_reason)
```
2. Element renders `m.policy.rule.*` state events in the policy rooms as the following, which in many cases are publicly readable by design.
```
@mjolnir:domain.tld created a rule banning users matching @some_user:domain.tld for some_reason
```

I think this feature could be useful to some and should probably be the default, but please make it configurable.

Start by tracing the spoiler behavior introduced by issue #559 and locating how moderation-room message formatting and bot configuration are handled. Done means the spoiler behavior is configurable, remains enabled by default, and the setting affects the relevant moderation messages.

Written by the indexing model from the issue text.