The gain, given the "real sites benchmark" below, is obvious:
```
benchmark old ns/op new ns/op delta
BenchmarkHugo-4 14497594101 13084156335 -9.75%
benchmark old allocs new allocs delta
BenchmarkHugo-4 57404335 48282002 -15.89%
benchmark old bytes new bytes delta
BenchmarkHugo-4 9933505624 9721984424 -2.13%
```
Fixes#2495
The current "rendering language" is needed outside of Site. This commit moves the Language type to the helpers package, and then used to get correct correct language configuration in the markdownify template func.
This commit also adds two new template funcs: relLangURL and absLangURL.
See #2309
