This adds a @pubsweet/base-model package, which brings a BaseModel class that components can extend
to add standalone date models to PubSweet. Each data model component can have its own GraphQL types
& resolvers, a JSON schema and migrations.

BREAKING CHANGE: In PubSweet server, the exported thing is no longer startServer directly, but it's
now part of the exported object. This will break applications that use the equivalent of const
startServer = require('pubsweet-server'). The new method for getting a startServer is const {
startServer } = require('pubsweet-server').

Closes #395