Poll fields colliding with poll preview

[nexo]

I saw this on Meta and it seems poll fields are overlapping the border of the poll preview.

[j.jaffeux (Joffrey Jaffeux)]

Thanks for report!

I made various tweaks to the UI:

@awesomerobot I let your merge it if you are satisfied:

https://github.com/discourse/discourse/pull/8985

[awesomerobot (Kris)]

Fixes it for me, merged.