Use default_url_options to generate absolute URLs

In the dev seeds, we were using `Setting["url"]/proposals`, but we can
use `proposals_url` instead, similar to what we do in the rest of the
application.

We can do a similar thing in the sitemap. This way the sitemap will also
work on installations who haven't manually set the "url" setting.

Since we aren't using `Setting["url"]` anywhere anymore, we're removing
it.

This setting was mainly redundant, since we already had the
`server_name` in the secrets. Furthermore, `server_name` is
automatically configured when running the installer, while
`Setting["url"]` had to be manually set in the admin section the
application was installed.

Note we're using `ActionMailer::Base` setting to generate URLs. Sounds a
bit strange, but it's a standard way Rails provides to generate URLs
outside the context of a request.

db/dev_seeds/settings.rb

@@ -18,7 +18,6 @@ section "Creating Settings" do
     "telegram_handle": "CONSUL",
     "twitter_handle": "@consul_dev",
     "twitter_hashtag": "#consul_dev",
-    "url": "http://localhost:3000",
     "votes_for_proposal_success": "100",
     "youtube_handle": "CONSUL"
   }.each do |name, value|