Widget Visibility: improvements to supported pages and conditions

Synopsis: quite a few improvements and bug fixes to the Widget Visibility module were proposed in the past year, but weren't merged. Merging all these in would offer significant improvements to the module, that could then be ported back to WordPress.com.

Project: https://github.com/Automattic/jetpack/projects/5

Responsible: None yet.

Technical approach: A few PRs were submitted, but now need a rebase. It might be best to create a new PR including multiple improvements, to make it easier to test and merge without having to deal with rebases, nor having to bother contributors who originally submitted the PRs but who may not have time to work on the project now.

Complete: 20%

Difficulty: 20

Last Updated: November 30, 2016 (by @jeherve)

Review project 5 and the earlier Widget Visibility pull requests mentioned in the issue to identify the proposed supported-page and condition changes. Consolidate the relevant improvements, rebase them onto the current code, and verify that the resulting changes are tested and ready for merging and later porting to WordPress.com.

Written by the indexing model from the issue text.