WIP - Migrate backend to Deno

[snowteamer]

Related to #586

Summary of changes:

• New Deno backend, using my fork of the Pogo server framework. An okTurtles-based fork could be used as well.
• All files in backend/ and shared/ have been converted to TypeScript.
• Some Flowtype definitions in shared/ were still necessary and have been moved to new types.flow.js files in their respective folders.
• Grunt tasks backend:launch and backend:relaunch have been replaced with deno:start.
• New deno:stop and flow:stop tasks, used to avoid dangling process issues.
• New exec:ts task for typechecking backend and shared files.

Additional info:

Known issues and limitation:

• Everything but the backend is still based on Nodejs. I think migrating one thing at a time is likely easier.

Remaining tasks:

• [x] Use Deno's --watch flag to implement npm backend script equivalent functionality without using Nodemon
• [x] Annotate backend files so they pass typechecking with deno check
• [x] Fix Flow errors in shared files
• [x] Replace Mocha by deno test for backend unit tests

[taoeffect]

Wonder if these updates to Deno are relevant? https://deno.com/blog/v1.35