Minor code changes extracted from #8151

[fregante]

What does this PR do?

• Extracted from https://github.com/pixiebrix/pixiebrix-extension/pull/8151

That PR is too big already, I removed some changes and sent them here:

1. Move a import config rule to out of the shared config
2. Fix potential page editor breaks on reload
3. Avoid involuntarily altering an input array via .reverse()
4. Extract selectElement from elementPicker, because that file is already big as it is

Checklist

• [x] Designate a primary reviewer: @grahamlangford