When using a html link checker with Hugo, this template consistently causes errors, as it renders `href=""` attributes when next/previous is disabled.
This change makes it so that the `href` attribute is not rendered at all if `HasNext` is false - which is better semantically, and makes link checking far easier.
* Rewind paginator for server mode
* Add some more related tests.
* Replace the clumsy scratch constructs in internal paginator template with variables
See #5825
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
