Upgrade to use metapackages

[isaacabraham]

Closes #557. This PR introduces dependencies on two NuGet metapackages, SAFE.Server and SAFE.Client. They contain the dependencies required for SAFE Stack applications as well as some nice helper types and functions.

• Only one dependency required for both Client and Server - less for end users to worry about.
• Opens the possibility of upgrades to existing SAFE Stack users "remotely" i.e. by releasing a new version of the metapackages - useful for e.g. security vulnerabilities etc.
• Allows people to more easily create their own SAFE Stack applications "from scratch".
• Adds some basic types and functions from e.g. Elmish Book as well as some helpers used internally at CIT.

I've also incorporated some of the types directly into the bookstore (notably, the AsyncOperation and Deferred types) (note that these will probably be renamed at some point to something a little more obvious).

I suggest that individuals test out this repository (or the SAFE.Meta packages) directly from this PR. Once we're happy with the shape of the metapackages, we can release it as a full v5 release (to go alongside SAFE Stack 5).

[isaacabraham]

@mattgallagher92 I'd like to get this merged in this week if possible - even if you have legitimate concerns about the API, we can rapidly iterate on that once the template is hooked up to using the package (and ideally published as a beta).

We can also "soft launch" the types and stuff (just including it in the package but not heavily publicising it) and iterating that way.