Add aria labels to toolbar

[clairefields15]

Adds aria-labels to improve the accessibility of toolbar elements and color picker dropdowns. Some existing unit tests had to be tweaked in order to add these aria-labels to the html markup but all tests pass.

addresses these issues:
#3360
#3320
#2038

[olsonjb]

@jhchen My team is interested in this one and it looks like you're the one with power to review around here - I'd love to see this one approved!

[clairefields15]

@luin looks like you've merged a few PRs recently, anything I can do to get this one moved along?

[joostverdoorn]

Would also love to see this happen! Anything I can do to help this? @luin

[ewalpole]

@clairefields15 I would love to see this merged but it looks like all of the labels are just English (maybe I am not understanding how it works??), I would need to be able to translate this to about 20 different languages in production so any idea on the best way to accomplish this? I can look at contributing as well.

[renoth]

I do not understand why this very useful and simple PR did not get merged yet, it adresses one of the most glaring issues with this editor that prevents our organisation from using it.

[ymushinsky-ms]

@clairefields15 hi, I am also looking for this functionality and appreciate the work you've done here, could you please fix the current conflicts for the merge? I'll try reaching out to @jhchen, @luin by email as well to try and get this merged :)

[TimaRebesh]

This issue is almost a year old. Are you going to solve it in the near future? Please, we are still waiting for

[luin]

Sorry for the delay. The commit is modified cherry-picked into the develop branch. It provides a basic support for aria-label but you are able to customize it by passing a HTML container as toolbar options as documented here. Thanks for the contribution!

I'm closing this now. Feel free to open new tickets if there are anything unaddressed.