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.
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