Loosen react/jsx-handler-names rule

Created by: mxstbr

This is a very stylistic rule that I don't think we should be warning about. I have a component like this:

<form onSubmit={this.fetchData} />

and I'm getting warnings in the console saying:

Handler function for onSubmit prop key must begin with 'handle'

We should be disabling this, because I think the above is perfectly fine even if not called "handleSubmit".