Add model for storing prepub timespan settings

[chigby]

Description

Fixes #2076

This PR adds a Wagtail settings class for customizing the display of timespan over which to display prepublication incidents. Here's what it looks like:

It occurs to me that maybe giving "hours" as a possible unit goes a little overboard, since right now we aren't resolving prepubs at that level (they only have a date, not a time), but maybe we will in the future? Easy enough to leave alone, I think.

This is not hooked up to anything, so there's nothing extremely substantial here. Just some settings and some help text. Feel free to offer feedback about any of it, though.

Type of change

• Bug fix
• New feature
• Vulnerabilities update
• Config changes
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires an admin update after deploy
• Includes a database migration removing or renaming a field

Testing

Honestly, just poke around in the settings, make sure they seem like they're doing the thing that the issue is asking for, maybe think about the editor UX, etc. Please let me know if I've overlooked something.

Post-deployment actions

In case this PR needs any admin changes or run a management command after deployment, mention it here: