DONKEY-561 Make the sandbox tree collapse button tabbable

[kailin-limble]

to conform to accessibility standards

https://limble.atlassian.net/browse/DONKEY-561

[slcwebguy]

question: did you check these updates across browsers by chance? What you have looks like it should work. I'm just wondering if maybe there are some different browser-specific implementations of unset (and/or revert) which would be problematic. I've not used those before myself.

[kailin-limble]

question: did you check these updates across browsers by chance? What you have looks like it should work. I'm just wondering if maybe there are some different browser-specific implementations of unset (and/or revert) which would be problematic. I've not used those before myself.

standard on all browsers except IE11