The `safeHTMLAttr` function operates on a full attribute definition, not
just within the attribute value.
Docs: https://gohugo.io/functions/safehtmlattr/
For `opengraph.html`, run the whole `content` HTML attribute through
`safeHTMLAttr`. That will preserve `+` signs in formatted dates.
For `vimeo_simple.html`, `safeHTMLAttr` was in the context of an
attribute value, thus having no effect. In this case we could replace it
with `safeURL`, but since the code is coming from an API it is safer to
just let Go's template engine sanitize the value as it already does with
`provider_url`.
Fixes#5236 (no need to change Go upstream)
Related to #5246
* Rewind paginator for server mode
* Add some more related tests.
* Replace the clumsy scratch constructs in internal paginator template with variables
See #5825
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 #5074Fixes#5763Fixes#5758Fixes#5090Fixes#5204Fixes#4695Fixes#5607Fixes#5707Fixes#5719Fixes#3113Fixes#5706Fixes#5767Fixes#5723Fixes#5769Fixes#5770Fixes#5771Fixes#5759Fixes#5776Fixes#5777Fixes#5778
The current full set of options for GA is now:
```toml
[privacy]
[privacy.googleAnalytics]
disable = false
respectDoNotTrack = true
anonymizeIP = true
useSessionStorage = true
```
Fixes#4775
