Upgrade Hugo to 0.87.0, and netlify-cli to v6

[chalin]

The following two features impact the page generation:

• Hugo v0.86.0: the render-link hook is now applied to all links, not just those written with the markdown syntax [...](...). For details, see markup/goldmark: Support auto links in render hook
  
• Hugo v0.87.0: accessibility improvement, markup: Add tabindex="0" to default <pre> wrapper

Modulo the changes induced by these two features, whitespace and date/time stamps, the generate site files contain only one diff:

--- a/blog/index.html
+++ b/blog/index.html
@@ -59,7 +59,7 @@
 				
 				
 				
-				<a class="nav-link active" href="/blog/" ><span class="active">Blog</span></a>
+				<a class="nav-link" href="/blog/" ><span>Blog</span></a>
 			</li>
 			
 			<li class="nav-item mr-4 mb-2 mb-lg-0">

Which is a bug, but this allowed me to note that we already had this bug, it's just now being more consistently applied -- i.e., the Blog top-nav entry isn't being shown as active when it should. For details, see #443.

[chalin]

This is the normalization script I used:

#!/bin/bash
#
# Make HTML files easier to diff.

set -x

pushd public

# Hugo version

find . -type f -name "*.html" \
     -exec perl -pi -e 's|<meta name="generator" content="Hugo .*?" />|<meta name="generator" content="Hugo X.Y.Z" />|' {} \;

# Ensure <meta ...> are on their own line
#
# Note: The while loop is necessary because s///g iterates only on the rest of the line, not the matched pattern (which is what we need).

find . -type f -name "*.html" \
    -exec perl -pi -e '1 while s|>(<meta [^>]+>)|>\n$1|g' {} \;

# Meta date or time

find . -type f -name "*.html" \
    -exec perl -pi -e 's!(<meta \w+="[^"]*(date|time).*?" content=").*?(" />)!$1HTML_NORMALIZED$3!' {} \;

# This is only for Hugo 0.86

# Strip all render-link hook additions

find . -type f -name "*.html" \
    -exec perl -pi -e 's| target="_blank" rel="noopener"||g' {} \;

# This is only for Hugo 0.87

find . -type f -name "*.html" \
    -exec perl -pi -e 's|pre tabindex="0"|pre|' {} \;

popd

[chalin]

Since this PR isn't introducing any new issues, I'm going to merge now. We can then address #443 for all top-nav entries.

/cc @nate-double-u