Replace util.LookUpHTML5EntityByName with html.UnescapeString

[jvoisin]

Golang has html.UnescapeString to unescape html entities. Would it be possible to make use of it instead of bundling a huge map? This accounts for 10% of the binary size of miniflux:

$ ls -lh miniflux.app 
-rwxr-xr-x 1 jvoisin jvoisin 26M Mar 13 23:40 miniflux.app
$ ~/go/bin/goweight  | head -n 8
   12 MB runtime
  8.1 MB net/http
  5.3 MB google.golang.org/protobuf/internal/impl
  4.1 MB net
  4.0 MB golang.org/x/sys/unix
  3.7 MB crypto/tls
  2.9 MB github.com/prometheus/client_golang/prometheus
  2.6 MB github.com/yuin/goldmark/util
$

cc @fguillot