Use the comment API rather than direct SQL queries in `comments_template()`.

`comments_template()` is used by most themes to display a post's comments. It
shows all comments that have been approved, and also shows all pending comments
by the current visitor (as determined by the comment cookies). However, the
comments API previously had no way of querying for "all comments that are
either approved, or are unapproved but written by foo@example.com". The
workaround was a direct SQL query: uncached, not subject to the same filters as
other comment queries, and just generally icky.

The new `include_unapproved` parameter for `WP_Comment_Query` accepts an array
of user IDs or email addresses. Pending comments associated with users in this
array will be included in query results, regardless of the value of the 'status'
parameter. In `comments_template()`, we leap from direct SQL queries to
`get_comments()` plus `include_unapproved', striving to put right what once
went wrong.

Props boonebgorges, simonwheatley, hardy101, jesin.
Fixes #19623.

git-svn-id: https://develop.svn.wordpress.org/trunk@29965 602fd350-edb4-49c9-b593-d223f7449a82