This commit replaces the main part of `helpers.StripHTML` with Go's implementation in its html/template package.
It's a little slower, but correctness is more important:
```bash
BenchmarkStripHTMLOld-10 680316 1764 ns/op 728 B/op 4 allocs/op
BenchmarkStripHTMLNew-10 384520 3099 ns/op 2089 B/op 10 allocs/op
```
Fixes#9199Fixes#9909Closes#9410