Implement auto-republishing of changed files in preview

[smaximov]

This PR adds a new interactive command op/do-publication-and-preview-watch. It does two things:

• publishes the site and starts preview (basically, calls op/do-publication-and-preview-site);
• starts watching the repository for changes. When any of watched files change, a watch handler publishes it again.

To remove all watch handlers, one should use command op/stop-all-watches.

[smaximov]

This feature can be used to implement live reloading.