Make replaceShortcodeTokens rewrite the input slice

Make replaceShortcodeTokens rewrite the input slice

Currently a `[]byte` copy is returned. In most cases this is the safe thing to do, but we should just modify/grow the slice as needed.

This is faster and consumes less memory:

```
benchmark                             old ns/op     new ns/op     delta
BenchmarkReplaceShortcodeTokens-4     7350          4419          -39.88%

benchmark                             old allocs     new allocs     delta
BenchmarkReplaceShortcodeTokens-4     5              1              -80.00%

benchmark                             old bytes     new bytes     delta
BenchmarkReplaceShortcodeTokens-4     4816          1152          -76.08%
```

This commit is aso a small spring cleaning of duplicated code in the different `PageConvert` methods.

Fixes #1516