Add a way to load with extension library open

[kyleplo]

Expected Behavior

There should be a way to load the extension library from the url, like https://beta.scratch.mit.edu/?extensions=all

Maybe also a way to load a specific extension, like https://beta.scratch.mit.edu/?extensions=music or multiple extensions like https://beta.scratch.mit.edu/?extensions=music,pen

Actual Behavior

This doesn't exist

Steps to Reproduce

Try it

Operating System and Browser

All

[benjiwheeler]

This sounds useful. Maybe @chrisgarrity and I could take a look in our URL work this month?

[thisandagain]

/cc @ericrosenbaum