Add status "awaiting user's verification for resolution" for WordPress.org support threads

[Daedalon]

WordPress.org support threads are currently often left by users in "not resolved" state even when commenting that the issue is fixed. "[ ] Mark this thread as resolved" below the comment box is not noticeable enough.

A new status should be added: "awaiting user's verification for resolution". This would be set by plugin support team members when they're providing user with an answer that is expected to solve the issue.

The comment box would then require the original poster to either accept or reject the resolution. The submission button would not be clickable before either one is selected.

The email notification that the original poster receives would also contain a request for action with links to either accept or reject the resolution. These would lead to the comment form that has the appropriate option selected.

[Daedalon]

This reduces the time plugin authors spend in manually setting tickets resolved after users confirm the resolution but forget to set the status. The less time spent on support bureaucracy, the more time spent on development and the stronger the WordPress ecosystem.

[samuelsidler]

Migrated to ​#meta165.