Signup: Fix username suggestions not working

[bisko]

Username suggestions were broken due to a prop name change in #10599.

This PR improves the fetching of the suggested username to use Redux and properly just pass the prop to SignupForm

To test:

Test free username:

1. Start Signup
2. Go through Signup
3. Select a domain name which would result a free username suggestion ( e.g. something123455123 )
4. Choose the free wordpress.com address
5. Reach the User step
6. See if the username field is properly filled up with the domain slug you wrote.

To test a taken username:

1. Start Signup
2. Go through Signup
3. Type kwight in the domain search box
4. Choose kwight.me
5. Choose a random plan
6. Reach the User step
7. See if the username is kwightweb or something different than kwight

Todo:

• The unit tests are broken for the PR. Will work on fixing them.

cc @kwight :)

[matticbot]

Test live: https://calypso.live/?branch=fix/signup-fix-suggested-username-not-appearing

[tyxla]

Works as described 👍

Code also looks good, I've just posted a couple of non-blocking nitpicks that you can feel free to skip and leave for another PR if you wish.

[tyxla]

Here we can use the single member import syntax, which is the recommended one recently:

import { get } from 'lodash';

[tyxla]

Dispatch will be mapped to action creators automatically, so:

( dispatch ) => ( {
	recordTrackEvent: ( event, data ) => {
		dispatch( recordTracksEvent( event, data ) );
	}
} )

can be shortened to just:

{ recordTracksEvent }

[michaeldcain]

LGTM