When inside front matter you specified series with spaces,
then the opengraph template wouldn't detect other articles,
because in `.Site.Taxonomies.series` they are stored by
urlized key.
Example:
```yaml
# in front matter
series:
- My Series
```
```gohtml
{{/* in a template */}}
{{- $series := index .Site.Taxonomies.series$name }}
{{/* was resolved to */}}
{{- $series := index {'my-series': ...} "MySeries" }}
```
.Lastmod is the time at which the website was most recently updated,
rather than .Date which is the time at which the website content file
was created.
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
Having them in separate files should make maintainance easier.
When adding new or making changes to the templates:
```bash
mage generate
```
This will get the Go code in sync.
Fixes#4457
