onboarding: "next" usability

[jamesacklin]

• Adds onSubmitEditing props to email fields so the user can press "enter" to submit the form via the system soft-keyboard
• Relocates the "Next…" button on signup and login screens to a hero button beneath the field

Signup screen:

Tlon login screen:

[linear]

TLON-3204 native: Tapping next on keyboard when email input is ready works

[latter-bolden]

Looks good, just some nits

• We have PrimaryButton which has a built in spinner
• Could probably simply leave it as Sign up and omit the signup method
• Feels a bit cramped visually, but whatever you see fit.
• Eula stuff should already be on develop (still seeing it in the diff)

[jamesacklin]

• Eula stuff should already be on develop (still seeing it in the diff)

I merged that PR and merged develop back in here, so that's reflected :)