To reduce the complexity of the linkcheck builder, this separates
the thread manager feature from the builder class as
HyperlinkAvailabilityChecker.

Move anchors_ignore, auth and to_ignore to
HyperlinkAvailabilityCheckWorker and become deprecated.

This is done by adding '\hskip0pt\relax\n' whenever a paragraph starts.

Add new configuration variables: `html_permalinks` and
`html_permalinks_icon`.

This refines the settings around HTML permalinks.

* html_add_permalinks
  * Deprecated.
* html_permalinks
  * Enable or disable permalinks feature.
* html_permalinks_icon
  * Change the icon for permalinks

The anchors for viewcode was generated in the reading phase only if
supported builder is used.  It causes anchors are missing on the
incremental build after the build for non supported builder.

This introduces `viewcode_anchor` node to insert the anchor even if non
supported builders.  They will be converted to the anchor tag in the
resolving phase for supported builders.  Or, they will be removed for
non supported builders.

These attributes were used to cache checked links and avoid issuing
another web request to the same URI.

Since 82ef497a8c88f0f6e50d84520e7276bfbf65025d, links are pre-processed
to ensure uniqueness. This caching the results of checked links is no
longer useful.

linkcheck: Do not check the availability of the same URL repeatedly

So far, linkcheck scans all of references and images from documents, and
checks them parallel.  As a result, some URL would be checked twice (or
more) by race condition.

This collects the URL via post-transforms, and removes duplicated URLs
before checking availability.

refs: #4303