Milestone widget: Make sure the widget ID is output

[dero]

The Milestone widget relies on the widget_id being exposed by the before_widget argument passed to register_sidebar in the theme. That is not guaranteed and the widget's live countdown is currently broken in many themes, most prominently in the current twentytwenty.

Changes proposed in this Pull Request:

• Create a custom ID value derived from widget_id and assign it to every widgets' .milestone-content container.
• In JS, target the newly assigned ID instead of the unreliable widget_id.

Is this a new feature or does it add/remove features to an existing part of Jetpack?

• This fixes a bug in the Milestone widget.

Testing instructions:

• On master, add the Milestone widget to one of the twentytwenty's Footer sidebars.
• Set the Milestone date and time to one minute in the future.
• On the front-end, open the browser's Dev tools and observe XHR requests being sent every second. (Aside: This feels like a DDoS waiting to happen, but I'll take a look into this separately.)
• Observe the countdown to not update.
• Switch to this branch.
• Refresh the page and set the Milestone time and date to one minute in the future again.
• On the front-end, observe the countdown being updated live.

Proposed changelog entry for your changes:

• Fix the live countdown feature of the Milestone widget in some themes.

[jetpackbot]

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

Scheduled Jetpack release: April 7, 2020.
Scheduled code freeze: March 31, 2020

Generated by 🚫 dangerJS against 9944ec0

[jeherve]

Do you think you could rebase that one now that we've merged #14864?

Thanks!

[dero]

@jeherve Sure, rebased in 9944ec0.