Followup Tasks for new Create Workspace page

[selfcontained]

Description

A few followup tasks from #16391 that added a new dedicated create workspace page. That page is still behind a flag, start_with_options, which is off everywhere. There were a few pieces of feedback on the initial review that this PR aims to address as an iteration.

• Shifts createWorkspace api call into a react query mutation. This allows us to drop tracking creating/error state manually.
• Handling closing of the existing workspaces modal (shifted it's rendering so it just overlays content instead of replacing too so it's less jarring)
• Adding "Cancel" button to existing workspaces modal for a consistent UX.

How to test

• Preview env: https://bmh-create09111b296b.preview.gitpod-dev.com/workspaces
• You'll need to have the start_with_options feature flag enabled for your user in non-prod.
• Clicking start new workspace on the Workspaces page should take you to the new create workspace page, or visit https://bmh-create09111b296b.preview.gitpod-dev.com/new/ directly.
• Ensure you can start a workspace by selecting a repo and hitting the New Workspace button.
• Try to start a 2nd workspace using the same repo (with the first one still running).
• You should see the Existing Workspaces modal. You should be able to close it now (couldn't before).

Release Notes

NONE

Documentation

Build Options:

• /werft with-github-actions
  Experimental feature to run the build with GitHub Actions (and not in Werft).
• leeway-no-cache
  leeway-target=components:all
• /werft no-test
  Run Leeway with --dont-test

Publish Options

• /werft publish-to-npm
• /werft publish-to-jb-marketplace

Installer Options

• with-ee-license
• with-slow-database
• with-dedicated-emulation
• with-ws-manager-mk2
• workspace-feature-flags
  Add desired feature flags to the end of the line above, space separated

Preview Environment Options:

• /werft with-local-preview
  If enabled this will build install/preview
• /werft with-preview
• /werft with-large-vm
• /werft with-gce-vm
  If enabled this will create the environment on GCE infra
• /werft with-integration-tests=all
  Valid options are all, workspace, webapp, ide, jetbrains, vscode, ssh

[werft-gitpod-dev-com]

started the job as gitpod-build-bmh-create-workspace-mutation.2 because the annotations in the pull request description changed
(with .werft/ from main)

[selfcontained]

/werft run with-preview=true

👍 started the job as gitpod-build-bmh-create-workspace-mutation.5
(with .werft/ from main)