Allow the resource URL to be overridden from the config file.

When cloning civicrm to a different path or domain, e.g. for testing/staging, the resource URL setting is a real pain. It con only be changed on-site, which is hard when the site is broken because it can't load any JavaScript. Even when going to civicrm/admin/setting/url directly it's hard to change, because, at least for me, the "save" button gets overlayed by the sidebar and can't be clicked. The problem arises again when moving a configuration from staging to production. This problem is keeping civicrm from being mobile in the sense of "copy&run".

It would be useful to be able to override the resource URL from the config file. The config file already defines CIVICRM_UF_BASEURL. There are several options:

• Use only CIVICRM_UF_BASEURL and abolish the on-site configurable URL. The URL directly relates to the server setup, there's really no good reason to change it online.
• Use CIVICRM_UF_BASEURL as the base, and make only the sub-path configurable on-site.
• Create an optional CIVICRM_UF_RESOURCEURL which overrides the setting from the database, if it's defined. If it is defined, the config page should display a warning the the setting is currently ineffective, and show instructions for removing the override.

This is closely related, but not quite the same as CRM-3716. I hope the problem is a bit clearer this time.