[6.x] Fixes UI inconsistencies

[kjetilhole]

This PR cleans up a few UI inconsistencies I noticed.

Fields

I found removing "Field" in title of fields to be easier to scan and better for translations.
The lighter background color now spans the entire width.

Before

After

Assets

When copying the URL of an asset, I fixed the position of the "Close" button to be centered.

Before

After

[ebeauchamps]

Interesting. In French, I took the decision to never translate field titles in order for users to easily find reference to those fields in the docs. However, "Field" is translated, which indeed brings some trouble as the French name "Champ" is placed after the name of the field (ie Bard Champ, User Group Champ, Color Champs.....Meanwhile, in "good" French, it should be placed before!). Therefore, I can confirm that deleting the "Field" part makes things easier to translate for those who want it and even easier for those who don't want it. Another solution could be for the gentlemen to add the "Field" part in each fieldtype title in order for translators to translate (or not) the whole key at once.