Enable more config options to link client code

[ZakarFin]

Adds more configuration options:

• Enable browser to request specific version of frontend with URL-parameter v=[version]. This is disabled by default but can be enabled in oskari-ext.properties:

# Enables client to request specific version of frontend - defaults to false
oskari.client.version.request=true

• Allow frontend code to be linked from another domain:

# For example "https://cdn.domain.com" if frontend files are served from another domain than the server - defaults to same domain
oskari.client.domain=

Customized/application specific JSP-files can use ${clientDomain} to enable the domain config. See the JSP changes here for details on how to use it.