Update directory/URL convention for extensions

It currently is not possible to compose links to extension resource files because the URL of the extension dir is unknown. Changes:

• Add config option, extensionsURL
• When constructing CRM_Core_Extensions, validate that extensionsURL has been set. If not, then warn the admin.

(NOTE: I considered setting a default value for directory/URL under 'files'. However, the technical details of setting a default for a dir/URL are too complex for a last minute patch.)