Change markdown rendering from blackfriday to goldmark (#9533)

* Move to goldmark

Markdown rendering moved from blackfriday to the goldmark.

Multiple subtle changes required to the goldmark extensions to keep
current rendering and defaults.

Can go further with goldmark linkify and have this work within markdown
rendering making the link processor unnecessary.

Need to think about how to go about allowing extensions - at present it
seems that these would be hard to do without recompilation.

* linter fixes

Co-authored-by: Lauris BH <lauris@nix.lv>

vendor/github.com/russross/blackfriday/v2/LICENSE.txt

@@ -1,29 +0,0 @@
-Blackfriday is distributed under the Simplified BSD License:
-
-> Copyright © 2011 Russ Ross
-> All rights reserved.
->
-> Redistribution and use in source and binary forms, with or without
-> modification, are permitted provided that the following conditions
-> are met:
->
-> 1.  Redistributions of source code must retain the above copyright
->     notice, this list of conditions and the following disclaimer.
->
-> 2.  Redistributions in binary form must reproduce the above
->     copyright notice, this list of conditions and the following
->     disclaimer in the documentation and/or other materials provided with
->     the distribution.
->
-> THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
-> "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
-> LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
-> FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
-> COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
-> INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
-> BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
-> LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
-> CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
-> LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
-> ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
-> POSSIBILITY OF SUCH DAMAGE.