batches: implement new URL scheme changes

Created by: courier-new

Follows from https://github.com/sourcegraph/sourcegraph/issues/28939, closes https://github.com/sourcegraph/sourcegraph/issues/29688! 🎉

Most of our pages were already accessible at the right paths, so here are the changes:

• /{user|orgs}/:ownerName/batch-changes/:name?tab=spec ➡️ /{users|orgs}/:ownerName/batch-changes/:name/executions
  • This is the details page with the batch spec executions list tab open.
  • ?tab=executions will also still work, but the idea is that this URL path would be more stable in the future even if the executions list view moves out of <Tabs />.
  • The legacy URL ?tab=spec is now reserved for when server-side execution is disabled.
  • I decided to leave the other tab URLs (e.g. ?tab=burndown ➡️ /burndown) alone for now because they'd be easier to change in the future and we would want to maintain backwards compatibility anyway.
• /batch-changes/executions/:executionID ➡️ /{users|orgs}/:ownerName/batch-changes/:name/executions/:executionID
  • This is the execution details page.
  • Because it now lives in the Org|UserNamespaceArea, I had to hack around that namespace header and narrow page layout again... 🙄 I'm looking forward to when we give every BC page the SSBC design treatment and have them bring their own page header. This should get significantly less complicated then.
• /batch-changes/executions/:executionID/edit ➡️ /{users|orgs}/:ownerName/batch-changes/:name/executions/:executionID/spec
  • This is the execution details page with the read-only spec focused.
  • In order to edit the spec, you would navigate back to /{users|orgs}/:ownerName/batch-changes/:name/edit. This workflow will become more obvious in EM3.5 once we incorporate the actual editor into this execution view.

Test plan

These changes are experimental behind a feature flag and should not impact existing customers, so I only went as far as to verify the intended functionality locally myself.