internal/history: split Release summary into bug and security fixes

Make it possible to use the higher-level fix summary fields, instead of
CustomSummary, when a release has both bug fixes and security fixes.

Rewrite the recent hand-written custom summaries to use the new fields,
which produces equivalent output that differs largely in white-space
and some trivial consistency fixes.

Fixes golang/go#51719.
Updates golang/go#38488.
Updates golang/go#44918.

Change-Id: I672cea21f63cb4ab9764efb6cbc783cf503b791c
Reviewed-on: https://go-review.googlesource.com/c/website/+/393357
Trust: Dmitri Shuralyov <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>

Author: dmitshur

_content/doc/devel/release.html

@@ -43,10 +43,20 @@ <h2 id="go{{.Version}}">go{{.Version}} (released {{.Date}})</h2>
 			{{.}}
 		{{else}}
 			{{if .Future}}will include{{else}}includes{{end}}
-			{{.Quantifier}}
-			{{if .Security}}security{{end}}
-			{{if eq .Quantifier "a"}}fix{{else}}fixes{{end -}}
-			{{with .ComponentsAndPackages}} to {{.}}{{end}}.
+			{{with .Security}}
+				{{.Quantifier}}
+				security
+				{{if eq .Quantifier "a"}}fix{{else}}fixes{{end -}}
+				{{with .ComponentsAndPackages}} to {{.}}{{end -}}
+			{{end}}
+			{{- if and .Security .Bug}}, as well as{{end -}}
+			{{if .Bug}}
+				{{.Bug.Quantifier}}
+				{{if .Security}}bug{{end}} {{/* If there are security fixes too, specify that what follows are bug fixes. */}}
+				{{if eq .Bug.Quantifier "a"}}fix{{else}}fixes{{end -}}
+				{{with .Bug.ComponentsAndPackages}} to {{.}}{{end -}}
+			{{end}}
+			{{- if or .Security .Bug}}.{{end}}
 			{{.More}}
 
 			{{if not .Future}}