Split client and server into independent packages

[c-w]

This pull request makes the server and client packages independently deployable and runnable. This makes the developer story slightly worse (we now have to run two servers in separate shells, see updates in readme), but it gives us a number of benefits, including:

• Better long-term maintainability by increasing the logical separation between client and server.
• Option to have client and server depend on different versions of the same package. This is a requirement for using GraphQL+Apollo since the Apollo React client and the Apollo Express server rely on different versions of the Apollo core library.
• Ability to publish client and server as independent npm modules, e.g. to deploy them to separate webapps or to include them selectively in third-party pages.

This pull request is a necessary pre-condition for a potential Fortis/Ibex integration, but has value on its own for the reasons mentioned above. As such, I propose to merge this pull request before we start the Fortis/Ibex integration.