Fit column width to child elements

[tmpacifitech]

Fixes plotly/dash-table#867

[akksi]

I think it's a nice approach to use flex instead of float here, the current solution has some alignment issues yet, a possible way to solve it:

1. Using flex instead of inline-flex as we should fill in the width of the column
2. adding justtify: space-between to keep the sorting icon aligned to the left and text to the right
3. removing float: left from the wrapper that prevents it from taking the width of the column
   (4. it would be nice to remove float: left from the children too, as it becomes pointless with this solution)

Current alignment:
Expected alignment:

[jackparmer]

@akksi @tmpacifitech is this one good to go?

[tmpacifitech]

@jackparmer Yes