Update github.com/google/safehtml to a version where it supports
embedding.

For golang/go#48410

Change-Id: I4efd6d270b38b0cc00647b3c1e87e46e5e0aab21
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/359394
Trust: Jonathan Amsterdam <jba@google.com>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Julie Qiu <julie@golang.org>
TryBot-Result: kokoro <noreply+kokoro@google.com>

The `pkgsite` command now embeds all the static assets it needs, so it
need not be told the location of the static/ directory with a
flag. The binary is self-contained and can be placed and invoked from
anywhere.

This required embedding the static/ and third_party/
directories. Since //go:embed cannot reference files outside the
containing package's tree, we had to add trivial Go packages in
static/ and third_party/ to construct `embed.FS`s for those
directories.

Also, the frontend needed to accept `fs.FS` values where it
previously took paths, and `template.TrustedFS` values where it
previously used `template.TrustedSources`. We ended up clumsily
requiring four separate config values:

- A TrustedFS to load templates.

- Two fs.FSs, one for static and one for third_party, to load other
  assets.

- The path to the static directory as a string, solely to support
  dynamic loading in dev mode.

For golang/go#48410

Change-Id: I9eeb351b1c6f23444b9e65b60f2a1d3905d59ef9
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/359395
Trust: Jonathan Amsterdam <jba@google.com>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Julie Qiu <julie@golang.org>
Reviewed-by: Jamal Carvalho <jamal@golang.org>