Present pipeline+mode as a tuple, lock pipeline mode in the playground

Summary:
This diff adds a feature flag that presents each pipeline+mode as a distinct item in the left nav and locks the mode selection within the pipeline tab.

The pipeline mode now appears in the URL all the time after the pipleine name, and I updated the helpers and convenience methods so the pipeline URLs are built in fewer places.

Todo:

• The quicksearch UI still shows individual pipelines
• The repo pipeline list still shows individual pipelines

Test Plan: Run tests

Reviewers: dish, alangenfeld

Reviewed By: dish

Subscribers: sandyryza

Differential Revision: https://dagster.phacility.com/D8071