Migrate to Typescript

[promaty]

Part of the effort to move all aragon apps to Typescript.
Since this will be significant amount of effort and we also have #35 open maybe we should just rewrite to Koa/Express to be consistent with other backend apps.

[sohkai]

Given that this was created for aragonOS DAOs, which will reach end-of-support (from A1) in the near future, we can defer this for a while.

If we create a notification backend for govern, that might be a good time to rewrite a fork of this in Koa/Express.