Validate content repository creation

[smashwilson]

The content repository creation form doesn't make any attempt to validate its inputs. There are a bunch of things we can give early feedback on without waiting until Docker tries to launch a container with bad configuration, such as:

• contentRepositoryPath and controlRepositoryLocation are both valid paths on the filesystem.
• contentRepositoryPath exists, is a readable directory, and contains a valid _deconst.json file.
• controlRepositoryLocation exists, is a readable directory, and contains a Gruntfile.js file.
• displayName is non-empty.

[smashwilson]

Oh, also, due to a VirtualBox quirk, you can only use directories beneath /Users as content or control repositories.

I should either:

• Warn about this during validation, or
• Do magic with symlinks to put them under /Users if they're not so you don't have to worry about it.