feat(lib/challenge): HTTP meta refresh challenge method (#623)

* feat(lib/challenge): HTTP meta refresh challenge method

Closes #95

This challenge method enables users that don't (or won't) support
JavaScript to pass Anubis challenges. It works by using HTML meta
refresh directives to ensure that the client is a browser.

This is OFF by default. In order to enable it, an administrator MUST
choose to make the default challenge method `metarefresh`.

TODO(Xe):

- [ ] Documentation on this challenge method
- [ ] Amend wording around Anubis being a proof of work proxy in the docs
- [ ] Add configuration file syntax for the default challenge method and settings
- [ ] Test with early customers

Signed-off-by: Xe Iaso <me@xeiaso.net>

* chore: spelling

Signed-off-by: Xe Iaso <me@xeiaso.net>

* fix(lib/challenge/metarefresh): use this value of err

Signed-off-by: Xe Iaso <me@xeiaso.net>

* docs: add metarefresh challenge info, Web AI Firewall Utility

Signed-off-by: Xe Iaso <me@xeiaso.net>

---------

Signed-off-by: Xe Iaso <me@xeiaso.net>

lib/challenge/metarefresh/metarefresh.templ

@@ -0,0 +1,17 @@
+package metarefresh
+
+import (
+	"fmt"
+
+	"github.com/TecharoHQ/anubis"
+)
+
+templ page(challenge, redir string, difficulty int) {
+	<div class="centered-div">
+		<img id="image" style="width:100%;max-width:256px;" src={ anubis.BasePrefix + "/.within.website/x/cmd/anubis/static/img/pensive.webp?cacheBuster=" + anubis.Version }/>
+		<img style="display:none;" style="width:100%;max-width:256px;" src={ anubis.BasePrefix + "/.within.website/x/cmd/anubis/static/img/happy.webp?cacheBuster=" + anubis.Version }/>
+		<p id="status">Loading...</p>
+		<p>Please wait a moment while we ensure the security of your connection.</p>
+		<meta http-equiv="refresh" content={ fmt.Sprintf("%d; url=%s", difficulty, redir) }/>
+	</div>
+}