Rename published_at and timestamp_for_sorting

published_at -> major_change_published_at
timestamp_for_sorting -> public_timestamp

Clarifying the use of each column
Add first_public_at to editions

Also only set first_published_at for some documents.

Saving first_published_at doesn't make any sense for consultations,
speeches and publications. This is due to use using a different date for
their first_public_at. Having the extra dates was confusing.
Remove last_significantly_changed_on as its unused
Use public_timestamp everywhere.

This should switch from using any other timestamp to using
public_timestamp in all view code.

The only exceptions for this are the change notes which also uses
first_public_at. And the feeds which also use first_public_at.

I have also remove the functional tests for change notes and am using
one cucumber test instead. It should better mock the flow of adding
change notes and seeing them apear on the frontend.
Remove unused headline partial
Remove first_published_at for some types

This should remove the data in `first_published_at` for consultations,
publications and speeches. It is currently populated with
`major_change_published_at` for the first major edition of the
respective document.

Having data in the column is confusing as we won't be putting data in
for new editions. For those edition types the `first_public_at` method
pulls data from different columns.
Send `public_timestamp` with filter results.

Don't expose `updated_at` to the frontend. Instead send
`public_timestamp` as its the date for displaying.