Update broken link icon to work in FontAwesome 4 and 5

[jasongrout]

#3475 fixed the broken link icon for FontAwesome 5, but unfortunately things broke for FontAwesome 4 (i.e., classic Notebook). This uses CSS that works for both FontAwesome 4 and FontAwesome 5, so the broken link works in classic notebook and JupyterLab.

[github-actions]

👈 Launch a binder notebook on branch jasongrout/ipywidgets/linkicon

[jasongrout]

@meeseeksdev please backport to 7.x

[lumberbot-app]

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

git checkout 7.x
git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

git cherry-pick -x -m1 575ce74c8b53764d6a8173d1c709fae27c8c9ec6

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

git commit -am 'Backport PR #3476: Update broken link icon to work in FontAwesome 4 and 5'

4. Push to a named branch:

git push YOURFORK 7.x:auto-backport-of-pr-3476-on-7.x

5. Create a PR against branch 7.x, I would have named this PR:

"Backport PR #3476 on branch 7.x (Update broken link icon to work in FontAwesome 4 and 5)"

And apply the correct labels and milestones.

Congratulations — you did some good work! Hopefully your backport PR will be tested by the continuous integration and merged soon!

Remember to remove the Still Needs Manual Backport label once the PR gets merged.

If these instructions are inaccurate, feel free to suggest an improvement.

[jasongrout]

Backported to 7.x in #3495