Earlier, the event sent when an onboarding step (hotspot till now)
is marked as read generated an event with type='hotspots' and
'hotspots' named array in it.
This commit renames the type to 'onboarding_steps' and the array
to 'onboarding_steps' to reflect the fact that it'll also contain
data for elements other than hotspots.
This commit adds a new endpoint 'users/me/onboarding_steps'
deprecating the older 'users/me/hotspots' to mark hotspot as read.
We also renamed the view `mark_hotspot_as_read` to
`mark_onboarding_step_as_read`.
Reason: Our plan is to make this endpoint flexible to support
other types of UI elements not just restricted to hotspots.
Prakhar Pratyush
Anders Kaseorg