Legend width shouldn't be hardcoded

[bh9]

Describe the feature:
The legend width is hard coded to 150px. This causes problems for entries with long names and is compounded with long lists of entries by the scroll bar. While I appreciate that it should have an upper and lower bound, it shouldn’t be hard coded. It seems the current value was set to lock the width of the colour palette. I would replace width: 150px; (kibana/src/ui/public/vislib/styles/_legend.less line 44) with

min-width: 150px;
max-width: 250px;

Feel free to suggest different values for min and max width if you think these are wrong.

Note, this change will have different results in firefox than all other browsers because of https://bugzilla.mozilla.org/show_bug.cgi?id=1307693 (firefox incorrectly handles overflow-y: auto)

[thomasneirynck]

I reverted that fix, as it seems to have broken master. #16174.

Likely due to screenshot-tests no longer working. Will revisit.

[ghudgins]

this will be addressed in this issue (planned for 8.2 release) #123803