[UX] Allow bulk-add/remove of languages

[klonos]

We just got an urgent ticket to add 40-some languages to the Dept. of Health site. ...what a nightmare of tedious clicking! 😞 😓

Can we please convert the select in the form that allows adding languages to a multi-select instead, and allow people to tick checkboxes instead. That would allow multiple languages to be added with a single form submission.

While at it, perhaps also allow bulk removal of languages from the language listing page.

[findlabnet]

Absolutely 81% use case. Or 99%?

[klonos]

I think that most sites will ever enable anything from 2-3 to less than a dozen languages. Once they've done that, they might never revisit these admin pages again for the entire lifecycle of the site. But the current UI makes it a hellish UX when you deal with sites that need more languages.

[klonos]

...btw, this may need to wait until we have a way to allow checkboxes in multi-selects in core (#459 / #1800 / #3069).