feat: enable loading config fragments (#321)

* feat(config): support importing bot policy snippets

This changes the grammar of the Anubis bot policy config to allow
importing from internal shared rules or external rules on the
filesystem.

This lets you create a file at `/data/policies/block-evilbot.yaml` and
then import it with:

```yaml
bots:
- import: /data/policies/block-evilbot.yaml
```

This also explodes the default policy file into a bunch of composable
snippets.

Thank you @Aibrew for your example gitea Atom / RSS feed rules!

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

* fix(data): update botPolicies.json to use imports

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

* fix(cmd/anubis): extract bot policies with --extract-resources

This allows a user that doesn't have anything but the Anubis binary to
figure out what the default configuration does.

* docs(data/botPolices.yaml): document import syntax in-line

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

* fix(lib/policy): better test importing from JSON snippets

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

* docs(admin): Add import syntax documentation

This documents the import syntax and is based on the block comment at
the top of the default bot policy file.

* docs(changelog): add note about importing snippets

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

* style(lib/policy/config): use an error value instead of an inline error

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

---------

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

data/common/allow-private-addresses.yaml

@@ -0,0 +1,15 @@
+- name: ipv4-rfc-1918
+  action: ALLOW
+  remote_addresses:
+  - 10.0.0.0/8
+  - 172.16.0.0/12
+  - 192.168.0.0/16
+  - 100.64.0.0/10
+- name: ipv6-ula
+  action: ALLOW
+  remote_addresses:
+  - fc00::/7
+- name: ipv6-link-local
+  action: ALLOW
+  remote_addresses:
+  - fe80::/10

data/common/keep-internet-working.yaml

@@ -0,0 +1,10 @@
+# Common "keeping the internet working" routes
+- name: well-known
+  path_regex: ^/.well-known/.*$
+  action: ALLOW
+- name: favicon
+  path_regex: ^/favicon.ico$
+  action: ALLOW
+- name: robots-txt
+  path_regex: ^/robots.txt$
+  action: ALLOW