docs: Fix issue with tags which contain slashes

Apache wouldn't forward api calls which contain encoded slashes (%2F), which caused the api calls to tags with slashes (e.g. 'te/st') to fail with a 404 not found.

See:
- https://httpd.apache.org/docs/2.4/mod/core.html#AllowEncodedSlashes
- https://serverfault.com/questions/715242/encode-url-wihthin-url-apache-mod-proxy-proxypass/715902#715902

INSTALL-OLD.md

@@ -211,6 +211,7 @@ server {
 
     DocumentRoot "/srv/www/booru/client/public"
     FallbackResource /index.htm
+    AllowEncodedSlashes On
 </VirtualHost>
 ```