[6.x] Stop asset field controls overflowing in narrow fields

[lazerg]

In a narrow asset field, like a 33% width field on a small screen, the browse button, the upload hint and the counter badge don't all fit on one line. The hint has a zero flex basis, so it collapses to no width and its text spills out past the edge of the field. In English it just looks cramped, but in German the words are longer and the hint becomes unreadable.

The control bar now wraps, and the hint is sized from its longest word instead of a zero basis, so it drops onto a second line when it can't fit beside the button. Fields wide enough for everything are unaffected and still render on one line.

Fixes #15433

[jackmcdade]

Thank you!