Make Page an interface

The main motivation of this commit is to add a `page.Page` interface to replace the very file-oriented `hugolib.Page` struct.
This is all a preparation step for issue  #5074, "pages from other data sources".

But this also fixes a set of annoying limitations, especially related to custom output formats, and shortcodes.

Most notable changes:

* The inner content of shortcodes using the `{{%` as the outer-most delimiter will now be sent to the content renderer, e.g. Blackfriday.
  This means that any markdown will partake in the global ToC and footnote context etc.
* The Custom Output formats are now "fully virtualized". This removes many of the current limitations.
* The taxonomy list type now has a reference to the `Page` object.
  This improves the taxonomy template `.Title` situation and make common template constructs much simpler.

See #5074
Fixes #5763
Fixes #5758
Fixes #5090
Fixes #5204
Fixes #4695
Fixes #5607
Fixes #5707
Fixes #5719
Fixes #3113
Fixes #5706
Fixes #5767
Fixes #5723
Fixes #5769
Fixes #5770
Fixes #5771
Fixes #5759
Fixes #5776
Fixes #5777
Fixes #5778

1 files changed, 13 insertions, 3 deletions

diff --git a/langs/language.go b/langs/language.go
index d741b9978..14e3263ae 100644
--- a/langs/language.go
+++ b/langs/language.go
@@ -113,9 +113,19 @@ func NewLanguages(l ...*Language) Languages {
 	return languages
 }
 
-func (l Languages) Len() int           { return len(l) }
-func (l Languages) Less(i, j int) bool { return l[i].Weight < l[j].Weight }
-func (l Languages) Swap(i, j int)      { l[i], l[j] = l[j], l[i] }
+func (l Languages) Len() int { return len(l) }
+func (l Languages) Less(i, j int) bool {
+	wi, wj := l[i].Weight, l[j].Weight
+
+	if wi == wj {
+		return l[i].Lang < l[j].Lang
+	}
+
+	return wj == 0 || wi < wj
+
+}
+
+func (l Languages) Swap(i, j int) { l[i], l[j] = l[j], l[i] }
 
 // Params retunrs language-specific params merged with the global params.
 func (l *Language) Params() map[string]interface{} {