feat: add themeConfig.serviceWorker.updatePopup option (fixes #453)

[mysticatea]

Summary

Fixes #453.

This PR adds the theme config to show the popup UI which notifies that the cache of the service worker is updated.

This PR includes #462. The #462 adds the functionality to call ServiceWorkerGlobalScope#skipWaiting() from theme's Layout class. Then this PR adds the popup UI to the default theme additionally.

New config is:

module.exports = {
    serviceWorker: true,

    themeConfig: {
        serviceWorker: {
            updatePopup: true || { message: "New content is available.", buttonText: "Refresh" }
        },
    },
}

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

If changing the UI of default theme, please provide the before/after screenshot:

The border is $accentColor.

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

• Chrome 66
• Firefox 60 - sw-update event didn't notified and Ctrl+F5 activated new service worker.
• Safari
• Edge 42 (EdgeHTML 17) - sw-update event didn't notified and Ctrl+F5 activated new service worker.
• IE - it doesn't support service workers.

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature: See Need service worker update alert window. #453 and feat: e.skipWaiting() in sw-update event #462.
• Related documents have been updated
• Related tests have been updated: I'm not sure how to test the service worker upgrading in automation.

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

[ulivz]

👍

[ulivz]

Excuse me for the delay to review this PR, Thanks for the super great work!