Add Easy User Switching

[pento]

Gutenberg has suffered from not testing as non-admin users, adding user switching would encourage doing that more.

[johnbillion]

There's a whole bunch of developer-focused plugins that should be bundled with environments that are aimed at developing core. I know the VVV team were looking into doing this for theirs, not sure if it went anywhere.

Lists:

• https://make.wordpress.org/themes/handbook/get-involved/become-a-reviewer/
• https://github.com/Automattic/developer/blob/cff96e8acb44684db0fc167487314787de6a5098/developer.php#L77-L171
• https://johnblackbourn.com/wordpress-developer-plugins/