[Layouts] DRY up shared code in `application`, `bare`, etc. layouts

[garyhtou]

A few of our layouts have duplicated code. This includes the new login layout from #9056.

IIRC, Rails has nested layouts. maybe that's something we can use.

[manuthecoder]

Should this be a part of UI3 or maybe after we release that?

[sampoder]

IIRC, Rails has nested layouts. maybe that's something we can use.

Please!

[garyhtou]

Should this be a part of UI3 or maybe after we release that?

separate from UI3!

[garyhtou]

@manuthecoder, IIRC, you were working on this?

[manuthecoder]

yep! I’ll get this done soon. I was also wondering what the timeline for this looks like, especially since we’re planning to release UI3 in the near future - it might make sense to hold off on this until then. Let me know if you’d prefer I continue with it now though!

[garyhtou]

This can be after UI3— it'll make the merge conflicts easier.