storage manager: use canonical URL for image URLs.

Generate image URLs relative to the site's canonical URL instead of
relative to the domain of the current request.

This means that all subdomains of Danbooru - safebooru.donmai.us,
shima.donmai.us, saitou.donmai.us, and kagamihara.donmai.us - will use
image URLs from https://danbooru.donmai.us, instead of from the current
domain.

The main reason we did this before was so that we could generate either
http:// or https:// image URLs, depending on whether the current request
was HTTP or HTTPS, back when we tried to support both at the same time.
Now we support only HTTPS in production, so there's no need for this. It
was also pretty hacky, since it required storing the URL of the current
request in a per-request global variable in `CurrentUser`.

This also improves caching slightly, since users of safebooru.donmai.us
will receive cached images from danbooru.donmai.us.

Downstream boorus should make sure that the `canonical_url` and
`storage_manager` config options are set correctly. If you don't support
https:// in development, you should make sure to set the canonical_url
option to http:// instead of https://.

config/danbooru_default_config.rb

@@ -28,14 +28,17 @@ module Danbooru
       Socket.gethostname
     end
 
-    # The canonical url for the site (e.g. https://danbooru.donmai.us)
+    # The canonical root url for the site (e.g. https://danbooru.donmai.us).
+    # Images will be served from this URL by default. Change this to http:// if
+    # you don't support HTTPS. Protip: use ngrok.com for easy HTTPS support
+    # during development.
     def canonical_url
-      "https://#{hostname}"
+      "https://#{Danborou.config.hostname}"
     end
 
     # Contact email address of the admin.
     def contact_email
-      "webmaster@#{hostname}"
+      "webmaster@#{Danbooru.config.hostname}"
     end
 
     # System actions, such as sending automated dmails, will be performed with
@@ -148,10 +151,10 @@ module Danbooru
     def storage_manager
       # Store files on the local filesystem.
       # base_dir - where to store files (default: under public/data)
-      # base_url - where to serve files from (default: http://#{hostname}/data)
+      # base_url - where to serve files from (default: https://#{hostname}/data)
       # hierarchical: false - store files in a single directory
       # hierarchical: true - store files in a hierarchical directory structure, based on the MD5 hash
-      StorageManager::Local.new(base_url: "#{CurrentUser.root_url}/data", base_dir: Rails.root.join("public/data"), hierarchical: false)
+      StorageManager::Local.new(base_url: "#{Danbooru.config.canonical_url}/data", base_dir: Rails.root.join("public/data"), hierarchical: false)
 
       # Store files on one or more remote host(s). Configure SSH settings in
       # ~/.ssh_config or in the ssh_options param (ref: http://net-ssh.github.io/net-ssh/Net/SSH.html#method-c-start)