Move to details/summary disclosure when browsers fully support it #1247
Description
The twisties boxes on various panels of the UI can be changed to HTML 5.1 <details> tags and <summary> tags when browser coverage is adequate. We are dropping IE11 support soon so it'll just be waiting for Edge to implement it. This can remove a fair chunk of custom code we use here with current <fieldset> implementation (and we can benefit from the in-built accessibility of those tags instead of coding our own ARIA role/attributes).
https://caniuse.com/#search=details
https://developer.microsoft.com/en-us/microsoft-edge/status/detailssummary/