In the web app Group Settings page, show a warning when pre-mod is turned on or off

**The problem**
At this time, on the Group Settings page for any Private, Restricted, or Open group, you can change the:
- Group name
- Group description
- Group type (Private, Restricted, Open)
- Moderation type (pre-mod on or off)

When you go to change the group type you get a warning message. The reason for this is because this change will affect the annotation visibility for all annotations in the group, and is a significant change to how the group behaves.

Similarly, changing the Pre-mod status in a group also impacts how the group behaves, and should have a similar warning popup forcing the group Owner to confirm they want to make this change.

**Potential Solution**
Show a warning popup (specific language TBD) when pre-mod is turned on or off, similar to how we show a warning when group type is changed.

Start at the web app's Group Settings page and inspect how the existing group-type warning popup is triggered. Add equivalent confirmation for turning pre-mod on or off, with wording to be decided, and verify that the warning appears for Private, Restricted, and Open groups before the change is applied.

Written by the indexing model from the issue text.