Remove flow

[emmatown]

We've discussed it and we're going to remove Flow. Maintaining our usage of Flow has been causing more pain than the benefit we get out of it.

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[gautamsi]

can we create a branch/tag which has Flow (just before this merge)? it would be helpful to use them when we go for TypeScript (soon hopefully). lot of typings can be copied to TypeScript as is even in jsdoc format.

see #1373

[emmatown]

We don't really need to make a branch since we can look at the git history to get to a version of the repo with flow in it

[gautamsi]

A git tag would be helpful and easy to get to

[emmatown]

Just pushed a flow tag from what is currently on master.