Use better icon for "Write to Disk"

[jidanni]

In the green buttons, there is an odd symbol, "--o--". Maybe use "💾" as it seems to mean "write to disk" better. Or maybe just put the word "Save" or "Write" on that button.

[samwilson]

It looks like it's the git-commit Octicon: https://primer.style/design/foundations/icons/git-commit-24

I agree, a more save-like icon might be more communicative of the purpose.

[simon04]

Migrated to bootstrap-icon https://icons.getbootstrap.com/icons/save/