[dagit] Create workspace routes

Summary:
This is a first step in the plan to add more specificity to Dagit URLs. Create workspace-level pages to serve as index pages for repositories, namespaced to /workspace paths.

• /workspace, an overview of all loaded repositories
• /workspace/[repo@location]/pipelines, all pipelines in a repo
• /workspace/[repo@location]/solids, all solids in a repo
• /workspace/[repo@location]/schedules, all schedules in a repo
• /workspace/pipelines/[pipeline_name], either a redirect page (if only one matching pipeline name found across repos) or a disambiguation page with all matches displayed

These pages aren't currently linked anywhere in Dagit. There are several more disambiguation pages to create (solids, schedules) and I expect that it would probably be good to have a page with all partition sets for a given repo.

Screenshots and overall plan below.

Test Plan: View all pages above, verify proper rendering and behavior.

Reviewers: bengotow, alangenfeld, dgibson, prha

Reviewed By: bengotow

Differential Revision: https://dagster.phacility.com/D4821