Switch from flow to typescript

As we were going through #770 with @housseindjirdeh, we both emitted the desire of ditching flow in favor of typescript.

I personally struggled with flow while trying to enforce types, using VS Code. Validation sometimes takes a few seconds, and the whole thing seems a bit sketchy to me.

Of course, this is up for debate :)

Start by reviewing issue #770 and the project's current Flow and JavaScript setup to establish the migration scope and tooling requirements. Done means the team has agreed on whether to replace Flow with TypeScript and defined the work needed across the GitHub mobile client.

Written by the indexing model from the issue text.