Engineering: Simplify adding data step in Wizard

[lucasrodes]

Currently, we have several pages to add data steps into Wizard: Express, Meadow, Garden, Grapher.

This can be confusing at times, and can add some extra maintenance.

We could make a simpler app page, which condenses all these options into one:

data-step.mov

[lucasrodes]

Implemented in #3718 and #3726.

Highlights

• We now have a single app where one can choose to create Meadow, Garden and/or Grapher steps.
• The UX has been made more compact by arranging widgets in rows.
• Snapshot selection is smarter. It allows you now to pick multiple snapshot, not necessarily with the same short_name. IT only accepts existing snapshots.
• The instructions a posterior are shown in a cleaner way.
• Migrated codebase concerning old app pages (meadow, garden, grapher, express) to module etl_steps_old, which is ready to be deprecated!