The page images selection order as follows:
1. Page's images parameter, image resources are supported.
2. Page's image resources that naming in *feature*, *cover* or *thumbnail* pattern.
3. If no page images specified, then the first one of site's images will be used as the fallback, supports site resources.
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
