Update the "All URLs" site shortcut to insert `<all_urls>`

[twschiller]

Context

• *://* only matches http/https URLs
• Some iframes use srcdoc so have a about:srcdoc URL
• <all_urls> is a special match pattern to match any URL with a permitted scheme https://developer.chrome.com/docs/extensions/develop/concepts/match-patterns. For the purposes of running mods, we treat it as matching about:srcdoc: https://github.com/pixiebrix/pixiebrix-extension/blob/733719b0a8f55f9f3160b0279b09a103714edc36/src/bricks/available.ts#L25-L25
• Chrome's contextMenu API also supports <all_urls> for documentUrlPatterns
• By and large, all of our uses of *://* that aren't for sidebar should actually be <all_urls> (e.g., context menu, snippets, etc.). It's safe to have the site pattern be that for sidebars too though

Discussion

• Are there some site/permissions contexts where <all_urls> is not a valid value?
• Ideally, end users should not have to think about the distinction between <all_urls> and *://*