Move to GitHub-first sync process

[zpao]

After talking with @hellendag, we've decided that the best thing to help Draft continue to grow and be healthy is to move to a GitHub-first model. This should make it much easier to accept pull requests more freely and sync into Facebook in batches. There are still a few things to figure out process-wise but we can start from the tooling side.

External

• Update to latest build process - @zpao (Generate .js.flow files #389)
• Ensure linting and tests are dependable, match FB expectations, CI fails appropriately - @zpao
• Set up issue & PR templates (docs, React's) - @hellendag
• Document testing guidelines - @hellendag
• Document manual test plan - @hellendag

Internal

• Write sync script (can crib from React's)

There are surely some other things, we can add as we go.

[mitermayer]

wohooooo!! amazing!

[mitermayer]

Let me know if there is anything I can do to help

[sophiebits]

Is this done?

[zpao]

Effectively. shipit is off. I'll still make linting stricter.

[mitermayer]

is this done ? can we close this ?

[flarnie]

Not only was this completed, but I am now moving us back to FB first. See #1244