Fix data races in sorting and Reverse

Fix data races in sorting and Reverse

The custom sort functions used from the templates had some subtle data race- and related issues,
especially when used in the single page template.

This commit fixes this by making copies and protect the read and writes with a RWMutex.

The results are cached (it will typically be invoked *number of pages* times with exactly the same data).

This is, not surprisingly, also faster:

```
benchmark                           old ns/op     new ns/op     delta
BenchmarkSortByWeightAndReverse     14228         491           -96.55%

benchmark                           old allocs     new allocs     delta
BenchmarkSortByWeightAndReverse     1              0              -100.00%

benchmark                           old bytes     new bytes     delta
BenchmarkSortByWeightAndReverse     32            0             -100.00%
```

Fixes #1293