Fix very narrow nested fields

[sedubois]

I am not especially talented at CSS, but here is a fix for the very narrow layout which makes editing bigger nested fields such as textareas very impractical.

Before:

After:

The problem can also be seen on http://administrate-prototype.herokuapp.com/admin/products when editing some product (product meta tag meta title and meta description are very narrow, which would be problematic if those fields were textareas).

[pablobm]

I like your approach of max-widths based on rems rather than percentages. Thank you.

[pablobm]

Hm, I just noticed that the resulting commit of my merge was attributed to me instead of you. I guess it's because it was a squash commit...? Apologies for that. I'll ask.

[sedubois]

Thanks @pablobm, yes a squash would be a new commit, which seems correct to me. Thanks for merging 👍