Add ability to import external dependencies from the shared module

[maurizi]

Detailed Description

In #177 I tried to add types to a module in shared that were aliases of geojson types.

However, because the shared module is outside of the root directory when accessed from the server, it does not have a node_modules directory and TypeScript cannot find any external modules.

[maurizi]

I worked-around this in development by symlinking the node_modules directory for server into shared, but that doesn't seem like an ideal solution.

Some potential options are:

• https://classic.yarnpkg.com/en/docs/workspaces/
• https://www.typescriptlang.org/docs/handbook/project-references.html