Closes #429

"Add sprite/backdrop" buttons should have a discernible text

In order to comply to WCAG 2.0[1], buttons must have discernible text that
clearly describe the purpose, so that it can be used by screen readers.

[1] https://www.w3.org/TR/2016/NOTE-WCAG20-TECHS-20161007/H65

Fix AudioContext issue in Safari

Set costume when selected in costumes tab

Record a new sound from the mic

Revert "Explicitly create variables in the VM"

Add pop sound to all sprites without sounds

Refresh workspace when setting it to visible

Pin to `latest` for 3.0 modules

Deploy all branches but master to subdirectories

In case the remote one gets deleted for some reason. I already had a fork of gh-pages, which is why I'm using rschamp rather than LLK. This is intended to be temporary.

Use `comments` flag to disable comments in scratch-blocks

Make escape key exit library modals