Update CSS class names for lumino

[vidartf]

Resolves #3357.

I also made sure that the <DEPRECATED> tags formed open/close pairs for easier processing in the future.

As discussed in the dev meeting, this also renames some of the internal CSS class names to avoid conflicting with the phosphor prefix.

[github-actions]

👈 Launch a binder notebook on branch vidartf/ipywidgets/lm-css

[jasongrout]

Thanks!

[martinRenou]

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 7078bc549bb859682cda99adfb5d2e9126ac531d

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

git commit -am 'Backport PR #3366: Update CSS class names for lumino'

4. Push to a named branch:

git push YOURFORK 7.x:auto-backport-of-pr-3366-on-7.x

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

"Backport PR #3366 on branch 7.x (Update CSS class names for lumino)"

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.